Drs Laurent

Manasi, Sasha Lerys

Mrs John

A Moran Gift

Louise Chisholm Moran

Joyce F Menschel Gift

Joseph Pulitzer Bequest

I am truly indebted to many people over the past three years. Knowing how 2020 impacted all of our lives, I cannot imagine how difficult it must have been for everyone to take the time to help me. I have made it this far thanks to them.

First and foremost, I would like to express my deepest gratitude to my adviser Rémi Dreyfus. Through his guidance I developed a deeper appreciation of not just the what, but the how and why of research. Whenever I had doubts or questions about my work he corrected me, gave invaluable advice, and encouraged me to keep going -and in the process I realized that the limitations I had were mostly self-imposed and imagined. We were operating in different time zones for the most

Daeyeon Lee, who hosted me at UPenn, definitely helped my return to Philadelphia go as smoothly as possible. I felt at home right away, not just because of the similar equipment found at the LOF and in the Lee lab, but because everyone in the Lee lab was welcoming and eager to help. Many thanks to Joseph Rosenfeld for helping me find the products and materials I needed to perform my experiments; Alex Wu for his tips on UV curing. I am especially thankful for Jaehyun Kim's help with sample preparation and SEM imaging. And of course, the talks with Daeyeon and Jaehyun were very helpful in planning my experiments.

I appreciate Paul Heiney's training on the SAXS beamline at the LRSM.

Introduction

One of many items housed in the Metropolitan Museum of Art of New York City is an 1844 daguerreotype of the Ramesseum in Thebes, Egypt (fig. 1). However, the clarity of the image and the blue sky in the background suggest a much more contemporary photograph. Daguerreotypes were made by exposing silver-iodide covered surfaces to light, followed by development in mercury vapor. This resulted in the formation of silver-mercury nanoparticles, which scattered light and gave high-resolution images. To introduce colors to otherwise black-and-white photographs, overexposure was utilized. This led to a higher number of silver nucleation sites and thus to smaller silver nanoparticles, creating a blue sky. These properties can be attributed to plasmonic resonance: nanoparticles absorb certain wavelengths and scatter others, and their size and relative positions determine the observed color. Speaking of colors and nanoparticles: could nanoparticles be used as a visual aid, for instance, to detect changes in their environment -in other words, a sensor? The development of such a system is the main focus of my thesis. The system is simultaneously mechanochromic and plasmonic. It is mechanochromic, since mechanical stress applied on the system induces a change in color. The observed color is due to plasmonic resonance of gold nanoparticles incorporated in the sensor: as seen in daguerreotypes, the nanoparticles will absorb and scatter specific wavelengths depending on their size and relative positions. If we can succeed in producing well-controlled, non-polluting mechanochromic microcapsules, the potential applications are endless. We could use them to detect bacterial contamination, or cracks or damage in materials. But before that, we need a deeper understanding of the optomechanics and better control of the fabrication process.

The first chapter will present several existing examples of mechanochromic technologies, such as dyes, fluorophores, and plasmomechanics. The working principles of the sensor, plasmonic resonance and Pickering emulsions, will be discussed. We will compare the proposed sensor to the existing technologies and show its advantages.

The second chapter presents new syntheses of gold nanoparticles that are more environmentally friendly, and I will characterize the nanoparticles in terms of shape and size. We will also see how modifying certain synthesis conditions such as time or reductant concentration can affect the end result.

Chapter three will describe the preparation of a gold-stabilized Pickering emulsion via ultrasonication, and the polymerization of the monomers in the dispersed phase of the emulsion, to obtain gold-acrylate microcapsules. Different oil phases, initiators and pH will be tested, to see how each parameter influences the color and size distribution of the microcapsules. We will also see the main challenge of the ultrasonication method: polydispersity of the microcapsules.

In the fourth chapter, a complementary method of emulsification will be introduced: microfluidics. We we will see why operating at the microscale gives more control and reproducibility of the emulsions compared to ultrasonication. How do we prepare Pickering emulsions in a microfluidic chip? What are the criteria for stable droplets? How do we prevent coalescence of the produced droplets? I will describe the experiments performed to answer these questions. We will see how to treat hydrophobic surfaces and how to predict flow regimes and droplet properties such as size and speed in a microchip.

Finally, I will introduce an alternative method of polymerization: the old method relied on heat, but here we will use UV light to cure monomers. After reviewing the merits of UV curing, we will see how gold's scattering and absorption properties can affect proper UV curing, and how we can avoid gold-related problems during setup of the experiments.

Chapter 1

State of the art 1.1 Introduction

As more and more polymers are used in our daily lives, there is a need to check for any damage or changes in these materials, such as cracks and strain [1]. Quick and effective detection of any issues will help determine if a material can be used safely, or needs repair or replacement. Information on where and how much damage occurs in materials is important to developing better products.

Assessing any local stress however usually relies on invasive or destructive methods [2]. An ideal stress sensor should indicate local stress, in situ and in real time. Essentially, we would like a material that changes color in response to local stress. Such objects are called mechanochromic [3]. In this chapter I will give some examples of mechanochromic sensors, then present my project, and discuss its merits compared to existing technologies.

Mechanochromic sensors 1.2.1 Excimers

Fluorophores can form dimers known as excimers [4]. The collisional interaction between a fluorophore in its excited state and another in its ground state is called dynamic excimer formation. The excimer complex quickly dissociates to ground state monomers, emitting fluorescent radiation at a wavelength longer than that of the excitation energy. This is called excimer fluorescence. Crenshaw et al. reported that excimers could be used to detect mechanical damage in polymer blends [5] [6]. The authors prepared fluorophore monomers, namely cyano-substituted oligo-(p-phenylene vinylene) derivatives (cyano-OPVs). Using the property that excimers emit fluorescence at different wavelengths than their monomer counterparts, the authors were able to determine the relative proportions of excimers and monomers in their system: mechanically deformed regions of doped materials will tend to contain fewer excimers relative to monomers, and the overall fluorescence will be attributed more to the monomers (fig. 1.1). Measurement of strain was achieved with a rheometer connected to a photoluminescence spectrometer. This allowed simultaneous measurement of stress-strain and of emission of the monomers and excimers (fig. 1.2). The ratio of monomer to excimer emission was shown to be reversible when a triangular strain cycle was applied. However the ratio was only accurate for stresses greater than 1 MPa.

Excimer-based technology presents other problems as well. Phase separation can arise between dyes and polymers containing them [3]. Fluorophore emission is temperature dependent [7] and requires a high doping of materials.

Dyes

Capsules can be filled with dye, so that they are released when the capsules are ruptured. Li et al. encapsulated 2',7'-dichlorofluororescein (DCF) in polyurethane/poly-(urea-formaldehyde) (PU/PF) microcapsules [8]. The microcapsules are dispersed in an epoxy resin. When the capsules are broken, DCF is brought in contact with amine groups in the epoxy. DCF then changes from an acidic form to a basic one, and its color changes from yellow to orange (fig. 1.3 and 1.4).

While this method is easy to implement, and allows for the detection of small cracks (approx. 10 µm), this is not reusable, and more of a binary sensor: it can only tell you if there is damage or not. Color change is due to release of DCF. [8] Spiropyran and other mechanophores Reversible systems have been developed using mechanophores (compounds whose reaction is triggered by mechanical force), notably those based on spiropyran (fig. 1.5a). Under mechanical stress, colorless spiropyran undergoes a ring opening as its spiro C-O bond breaks, and becomes colored merocyanine [9]. Light reverts merocyanine to spiropyran. Depending on the attachment positions of spiropyran, either the spiro C-O bond or the spiro C-C bond is preferentially affected by stress. The spiro C-O bond is targeted if the attachment points are 5' or 6' on the indole side and 7 or 8 on the pyran side. Davis et al. used the 5' and 8 positions and showed that color change occured under tensile stress [10]. In addition, the intensity of the color and fluorescence increased with stress. The color of a polymer containing spiropyran translates the mechanical forces at play.

Using spiropyran does have its drawbacks. Synthesis of spiropyran is not simple. To properly function as a stress sensor, the molecule has to be first covalently bonded to the polymer of interest. [10] In 2018 Hu et al. introduced a concept named mechanochemically gated photoswitching [11]. A gated system is a system where chemical reactions occur only if a specific stimulus takes place. The authors synthesized a cyclopentadiene-maleimide Diers-Alder adduct that could be mechanically activated. After activation, the molecule became colorless under visible light and colored under UV light. This photoisomerization was reversible, allowing for on-demand evaluation of mechanical stress in materials. In addition the mechanophore was shown to be thermally stable.

One of the main appeals of this system is that inspection of damage can be done on-demand, since it is only revealed under UV light. However the synthesis is onerous and the molecule cannot be easily modified (low chemical modularity).

Photonic crystals

Photonic crystals are periodic dielectric structures exhibiting a photonic stopband that reflects light in a certain range of wavelength [12]. There are several advantages to using photonic crystals. Scattering-induced colors are much brighter than colors in pigments and do not fade over time [13]. There is no need of a power source to use the crystals either. Cho et al. made inverse opals as light-weight, mechanochromic sensors [14]. Inverse opals are basically negatives of opals: material is found where gaps would be and vice versa. The authors achieved this by infiltrating a crystal of silica particles with a polymer, SU-8. Then they removed the silica leaving only the SU-8 in place (fig. 1.7). Then, a spherical indenter was applied on the surfaces of the opals. Compression caused a change in opal configuration and thus in scattering. Elastoplastic deformation of SU-8 prevented the opals from returning to their original conformation, meaning the color change was permanent, giving a record of the mechanical forces applied on the opal (fig. 1.8). Compared to existing photonic crystals the inverse opals could record much higher loads (17.6-20.4 mPa instead of the usual 10-100 kPa [15]). The opals were highly sensitive. The ratio of shift in the stopband wavelength to the change in applied strain was up to 5.7 nm/%, the highest value compared to literature at the time.

One must be careful when making photonic crystals, however: defects in the crystals can cause changes in the reflectance. Figure 1.8 -Optical microscopy images of inverse opals with a diameter of a,d,g) 320 nm, b,e,h) 285 nm, and c,f,i) 238 nm after indentation at a,b,c) 30 mN, d,e,f) 60 mN, and g,h,i) 90 mN. j) The letters consisting of three different spots indented at 30, 60, and 90 mN. Reflectance spectra of inverse opal structures after indentation with a particle diameter of k) 320 nm, l) 285 nm, and m) 238 nm. The force was applied by a spherical indenter with a diameter of 301 µm at a fixed loading rate of 100 mN/s. [14] Plasmomechanics A more recent type of mechanochromic sensors are plasmomechanics [16]. The mechanism, plasmon resonance, will discussed in detail later, as it is the heart of the technology of this thesis, but essentially, depending on the distance between coupled metallic particles, they will absorb different wavelengths of incident light, and thus a different color will be visible.

Cataldi et al. developed a system in which gold nanoparticles (AuNPs) are embedded in a polymer tape (polydimethylsiloxane, PDMS) [17]. Pulling on the tape caused the distance between AuNPs to increase and change the overall color of the tape (fig. 1.9). The authors were able to express extinction spectra with respect to the applied strain (fig. 1.10), showing that plasmonic materials could be used as highly sensitive optical sensors.

Since small changes in interparticle distance have a large effect on coupling, when building plasmomechanic systems, one must be able to control the sizes of the particles used and their interparticle distance. Extinction spectra acquired while increasing the strain from 0% to 20.8% on PDMS film (after twelve growth cycles). (d) Extinction spectra acquired while decreasing the strain from 20.8% back to 0% . [17]

Comparing technologies

Let us briefly review the pros and cons of using the aforementioned mechanochromic systems. This is not an exhaustive list of course, but we should keep in mind that a variety of options exist. Depending on our needs, we can choose one method over the others.

Type

Advantages Disadvantages Excimers Taking these properties into account, the COMPASS group (collaboration between Solvay, CNRS and Upenn; Philadelphia, PA, USA) has developed their own mechanochromic sensors in the form of plasmonic microcapsules [18]. They are easy to prepare in large quantities, and can be made to be sensitive to mechanical strain or pH. The plasmonic resonance gives intense colors. Color change is sensitive and reversible. The microcapsules are elastic, so they can relax when strain is no longer applied. This also means that they are reusable. Each capsule is its own sensor: embedding capsules into a material can provide a highly localized response. The capsules are long-lasting too, as they are made from Pickering emulsions, which are very stable. I will discuss the mechanisms that make these microcapsules possible in the following sections.

Surface plasmon resonance 1.3.1 Principle

Free electrons of nanometric scale metals, when subjected to an external electromagnetic field, oscillate collectively (fig. 1.11). The oscillations, known as plasmons, occur at well-defined frequencies, since the free electrons are confined. This makes plasmons a type of bosonic quasiparticle excitation [19] [20]. Plasmons on the surface of the metal will absorb incident light that oscillates at the same frequency as themselves, reflecting the rest of the spectrum. This is called surface plasmon resonance (SPR). Depending on the resonance wavelength, different colors can be observed. The phenomenon is manifest in many works of art such as stained glass from the Middle Ages, which contain gold colloids that give a red color, and silver colloids that give a yellow hue (fig. 1.12). Most plasmonic applications are gold or silver-based since they resonate in the visible or NIR spectra [21], but other examples of plasmonic metals include aluminum [22] and copper [23].

Figure 1.11 -Surface plasmon resonance. [24] Figure 1.12 -Different colors of stained glass are due to the size and shape of metal colloids. [25] To better understand the differences seen in surface plasmon resonance, Mie theory can be applied to nanoparticles.

Mie theory

SPR of small particles in particular can be studied using Mie's solutions to Maxwell's equations. According to Mie theory, the extinction crosssection σ of particles of radius R much smaller than the wavelength of incident light λ, such that 2R λ and 2R < 20 nm is given by the following equation:

σ(ω) = 9 ω c ε 3/2 m V 4πR 3 3 ε 2 (ε 1 + 2ε m ) 2 + ε 2 2 (1.1)
where

                             ω
is the angular frequency of the exciting light c is the speed of light V is the volume of the particle

ε m is the dielectric constant of the surrounding medium ε(ω) = ε 1 (ω) + iε 2 (ω)
is the dielectric function of the particle itself ε m is considered to be frequency independent. ε 2 is energy dependent. Resonance is achieved when:

ε 1 (ω) = -2ε m (1.2)
Beer-Lambert law is related to the cross-section [26]. Consider a solution of thickness dl. Light intensity dI absorbed per dl is proportional to the intensity of incident light I, n, the number of molecules per cm 3 , and σ:

dI dl = -Iσn (1.3)
Applying boundary condition I = I 0 at l = 0 to eq. 1.3 gives ln

I 0 I = σcl (1.4)
where c is the concentration (in mol/L) and l is the thickness of the sample. This can also be expressed as:

log I 0 I = nl (1.5)
where is the decadic molar exctinction coefficient (in M -1 cm -1 ). Since n = N A C/10 3 (where N A is Avogadro's number), by combining eqs. 1.4 and 1.5, we can establish:

σ = ln10 10 3 N A (1.6)
Absorbance A can be measured with UV-vis spectrometry. According to the Beer-Lambert law:

A = σN A Cl 10 -3 ln10 (1.7)
where C is the molar concentration of the metal being measured and l the optical path length.

Factors that affect surface plasmon resonance

Elements According to eq. 1.1, σ depends on the dielectric function of the material being studied. The real part of the dielectric function ε 1 and the imaginary part ε 2 are responsible for the resonance peak position and bandwidth, respectively. For example, gold and silver nanoparticles of similar size yield different colors and UV-vis spectra (fig. 1.13).

Size

Once again the eq. 1.1 and fig. 1.12 show how size determines SPR properties. Liz-Marzan et al. studied various sizes of gold nanoparticles (AuNPs) and showed that the larger the AuNP, the higher the resonance wavelength (i.e. more red-shifted, see fig. 1.14a). Plasmon bandwidth however decreases with increasing size. This can be explained by the fact that electrons can reach the surface of small particles faster than that of large ones, resulting in quicker scattering. Resonance peak intensity increases with particle size, as the intensity is proportional to radius cubed (fig. 1.14b). Normalization is performed at 400 nm, which is the wavelength at which only atomic gold (Au 0 ) absorbs.

Shape

The shape of the NPs greatly influences the resonant bandwidth and peak position. Chen et al. showed that the shape of AuNPs not only in-fluences peak position, but also peak number: rod-like AuNPs possess two axes, and thus two oscillation modes. In terms of UV-vis spectra, this is shown by the presence of two peaks. The lower wavelength corresponds to transverse plasmons, and the higher wavelength corresponds to longitudinal plasmons. Transverse plasmon peaks are located at similar wavelengths to the peaks of spherical AuNPs, but longitudinal plasmon peaks depend on the aspect ratio of the AuNPs (fig. 1.15b).

Surrounding medium

The refractive index of the medium surrounding NPs has an effect on the extinction (fig. 1.16) [31]. Figure 1.16 -Extinction of 50 nm-large AuNPs according to surrounding medium. [31] Distance between nanoparticles Resonance of coupled plasmonic particles is also shown to strongly depend on interparticle distance. Dolinnyi used Mie theory to calculate extinction spectra of gold nanosphere (diameter 40 nm) pairs for several values of interparticle distance s (fig. 1.17) [32]. When s tends to infinity, the extinction spectrum exhibits a single peak. For s ≈ 7 nm, the plasmon band widens and the peak is red-shifted (fig. 1.17b). For intermediate values of s (0.4 s 4 nm) (c-f) two modes appear. The peak of the first mode does not depend on s but the second mode's peak increases in wavelength and decreases in intensity with decreasing s. A third mode is observed near 620 nm when s < 0.4 nm (g). We have seen how different factors affect the plasmon resonance and extinction spectra of AuNPs. And as seen in section 1.2, plasmonic resonance provides highly sensitive and robust signals. This is why we based our mechanochromic system on plasmons. AuNP synthesis and comprehension of the plasmon resonance is only the first step. If we want intense colors, the NPs should be closely packed. If we want a reversible color change and/or reusable system, we should envision an elastic medium for our NPs.

To address these issues COMPASS developed mechanochromic microcapsules, based on Pickering emulsions. The principles of Pickering emulsions and examples of microcapsules will be introduced, followed by a presentation of COMPASS's system.

Plasmon-based mechanochromic microcapsules 1.4.1 Pickering emulsions

In general, emulsions involve surfactants. However, as Pickering [START_REF] Spencer Umfreville | Cxcvi.-emulsions[END_REF] and Ramsden [START_REF] Ramsden | Separation of solids in the surface-layers of solutions and 'suspensions' (observations on surfacemembranes, bubbles, emulsions, and mechanical coagulation)[END_REF] independently reported, it is possible to create emulsions that are stabilized by solid particles. Such emulsions are called Pickering emulsions. Interest in Pickering emulsions was relatively small until the work of Velev et al. in which the authors showed latex particles could be used to create stable oil-in-water emulsions [START_REF] Velev | Assembly of latex particles by using emulsion droplets as templates. 1. microstructured hollow spheres[END_REF]. Since then, Pickering emulsions have been applied to fields including cosmetics [START_REF] Tang | Stimuliresponsive pickering emulsions: recent advances and potential applications[END_REF], food science [START_REF] Norton | Designer colloids-towards healthy everyday foods?[END_REF], oil recovery [START_REF] James R Bragg | Oil recovery method using an emulsion[END_REF][39], and pharmaceuticals [START_REF] Simovic | Nanoparticle layers controlling drug release from emulsions[END_REF]. As varied as the applications are the natures of solids used as stabilizers. Examples include silica [START_REF] Frelichowska | Pickering emulsions with bare silica[END_REF], latex [START_REF] Binks | Pickering emulsions stabilized by monodisperse latex particles: Effects of particle size[END_REF], and gold [START_REF] Bollhorst | Colloidal capsules: nano-and microcapsules with colloidal particle shells[END_REF].

Principle

Wettability of the stabilizer is conducive to the formation of either oilin-water or water-in-oil Pickering emulsions. Studying the contact angle of a spherical particle at a liquid/liguid interface, θ, helps understand and predict the outcome (fig. 1.18). Let us consider an emulsion comprising equal parts oil and water. The stabilizers are spherical particles. If θ < 90 • , then an oil-in-water emulsion is favored, and vice versa. Extreme values of θ mean unstable emulsions, with small θ meaning high hydrophilicity and large θ meaning high hydrophobicity, since the particles will tend to stay in their preferred phase. The volumic ratios can affect the nature of the emulsion as well. Increasing the volume of the dispersed phase can cause it to become the continuous phase in what can be called "catastrophic phase inversion" [START_REF] Binks | Catastrophic phase inversion of waterin-oil emulsions stabilized by hydrophobic silica[END_REF]. The value of θ can be derived by Young's equation (eq. 1.8),

cos(θ) = γ o/s -γ w/s γ w/o (1.8)
where

    
γ o/s is the surface tension between the oil and the solid γ w/s is the surface tension between the water and the solid γ w/o is the surface tension between the water and the oil Particles that adsorb to oil/water interfaces decrease the interfacial free energy since the contact area between oil and water is occupied by particles instead. The difference in free energy caused by the adsorption of a particle of radius r can be expressed as follows. ∆E = -πr 2 γ o/w (1 -|cos(θ)|) 2 (1.9)

If the minimum of free energy is very large compared to k B T the particles remain at the liquid/liquid interface and stabilize the emulsion. According to equation 1.9, when θ=90°the energy gain becomes maximal. A micronsized particle at a typical oil/water interface (γ o/w ≈50 mN/m) with θ=90°i s trapped in an energy well of roughly 10 6 k B T [START_REF] Levine | Stabilization of emulsions by fine particles i. partitioning of particles between continuous phase and oil/water interface[END_REF].

Factors that affect Pickering emulsion formation

Pickering emulsion formation depends primarily on the morphology of the particles, the surface tension γ o/w and the hydrophilicity of the particles.

The smaller the particles, the smaller the energy gain (eq. 1.9). Therefore, larger particles will be favored at the interface. Emulsion stability increases with particles' aspect ratio, too, because of stronger capillary attraction [START_REF] Madivala | Exploiting particle shape in solid stabilized emulsions[END_REF].

Oil polarity dictates whether an emulsion will be oil-in-water or waterin-oil. Binks and Lumsdon [START_REF] Binks | Effects of oil type and aqueous phase composition on oil-water mixtures containing particles of intermediate hydrophobicity[END_REF] showed that partially hydrophobic silica particles gained hydrophobicity at polar oil/water interfaces (e.g. esters, alcohols) and preferred water-in-oil emulsions, with large values of θ. The same particles became relatively hydrophilic at non polar oil/water interfaces (e.g. alkanes), with smaller θ. Also, when the water/oil interface is negatively charged, electrostatic repulsion hinders the adsorption of particles [START_REF] Marinova | Charging of oil-water interfaces due to spontaneous adsorption of hydroxyl ions[END_REF] [START_REF] Mirjam | Electrostatics at the oil-water interface, stability, and order in emulsions and colloids[END_REF].

There are many ways to change the wetting properties of particles. For instance, functionalizing hydrophilic particles with surfactants of opposite charge can make them hydrophobic [START_REF] Giermanska-Kahn | Particle-stabilized emulsions comprised of solid droplets[END_REF]. Changing pH is another way to modify wetting of the particles. If particles have ionizable groups at their surface, increasing pH will make the particles more hydrophilic [START_REF] Lambertus | Contact angles on charged substrates[END_REF] [START_REF] Binks | Inversion of emulsions stabilized solely by ionizable nanoparticles[END_REF]. Binks and Lumsdon showed this for partially hydrophobic silica particles (fig. 1.19) [START_REF] Binks | Effects of oil type and aqueous phase composition on oil-water mixtures containing particles of intermediate hydrophobicity[END_REF]. When pH<9, particles are preferentially dispersed in oil, while at pH>12.5, they are more dispersed in water. At the surface of the silica particles, SiOH are deprotonated to become SiO -. Low pH results in water-in-oil emulsions, and high pH results in oil-in-water emulsions. Gautier et al. reported that the mean surface coverage of latex stabilized emulsions varies with pH [START_REF] Gautier | Pickering emulsions with stimulable particles: from highly-to weakly-covered interfaces[END_REF]. At pH=1, a close-packed monolayer of particles formed at the interface, whereas at pH=4.8 the emulsion droplets are not homogeneously covered (fig. 1.20). Salts present in the emulsion can also affect the stability of Pickering emulsions. Golemanov et al. showed that stability of water-in-oil emulsions stabilized by latex particles was improved with the addition of electrolytes, in the form of NaCl [START_REF] Golemanov | Latex-particle-stabilized emulsions of antibancroft type[END_REF]. The electrolytes suppressed the electrostatic barrier to particle adsorption, which led to easier particle adsorption and stabler emulsions. The authors did observe aggregation of the particles in aqueous solution when NaCl concentration was increased, prior to emulsification. In the case of non polar oils, despite the aggregation, a stable emulsion was still obtained, and large latex aggregates were not observed in the emulsion. The authors hypothesized that the bulk aggregates adsorbed, then released particles, and a result the aggregates shrank. When the oil was polar, aggregates would simply move from the aqueous to the oil phase. The authors also considered particle-to-drop attachment as a type of coagulation and applied the Schulze-Hardy rule to emulsification. The Schulz-Hardy rule states that critical coagulation concentration (CCC) scales as 1 z 6 where z is the valence of the counterion (here, the electrolyte) [START_REF] Trefalt | Schulze-hardy rule revisited[END_REF]. For example, testing 1/3 3 =0.00137 M AlCl 3 led to a stable water-in-oil emulsion.

As we have just seen, if we want to then promote a certain type of emulsion (oil-in-water or water-in-oil), we have a number of ways to predict and steer the behavior of the particles by either changing their surfaces or changing their medium (pH, polarity of oil phase, concentration of electrolytes).

Interactions

Interactions, both repulsive and attractive, are present in Pickering emulsions. The forces involved dictate the behavior of the particle stabilizers.

In the water phase, charged particles attract groups of opposite charge. This leads to electrostatic repulsion between neighboring particles at the interface. In the oil phase, water may be trapped at the surface of the particles, and this can also lead to repulsion (fig. 1.21a) [START_REF] Mcgorty | Colloidal self-assembly at an interface[END_REF].

Pieranski's findings, which involved water/air interfaces [START_REF] Pieranski | Two-dimensional interfacial colloidal crystals[END_REF], have been applied to oil/water interfaces as well. When particles are at the interface, their surface charges become non homogeneous and create dipoles. The particles then repel each other (fig. 1.21b). Van der Waals forces are also in play, and they depend on the distance between particles and on θ. When the interparticle distance is on the order of nanometers, van der Waals forces are dominant [START_REF] Nikolaides | Colloids at liquid-liquid interfaces[END_REF].

Capillary forces depend not only on the aspect ratio of the particles, but also on their size and density, as shown by Chan et al. [START_REF] Chan | The interaction of colloidal particles collected at fluid interfaces[END_REF]. Capillary forces can be either attractive or repulsive. Large or dense particles deform the interface and capillary forces become non negligible [START_REF] Mcgorty | Colloidal self-assembly at an interface[END_REF]. If the particles' diameter is <1 µm then capillary forces are negligible [START_REF]Colloidal Particles at Liquid Interfaces[END_REF]. These properties can be expressed via the Bond number, which is a dimensionless number representing the ratio between gravitational and capillary forces, and can characterize shapes of bubbles and drops in a fluid [START_REF] Wang | Instantaneous bond number for a particle detaching from a bubble[END_REF]. It can be written as such:

Bo = ( L λ c ) 2 (1.10)
where L is the characteristic length and λ c = γ ρg is the capillary length, with:

     γ is surface tension ρ is mass density g is gravitational acceleration
In summary, Pickering emulsions are stable and the adsorption properties of the solid stabilizers can be tuned by modifying their size, shape and charges. We will now see microcapsules based on Pickering emulsions.

Syntheses of microcapsules

Colloidosomes

Microcapsules have been made using Pickering emulsions. The first instance of microcapsules is attributed to Velev et al. who used latex particles and octanol-in-water emulsions [START_REF] Velev | Assembly of latex particles by using emulsion droplets as templates. 1. microstructured hollow spheres[END_REF]. The latex particles were first functionalized with lysine hydrochloride. Lysine adsorbs on the negative charges of the latex particles and renders them partially hydrophobic. HCl and CaCl 2 , flocculating agents of latex, were added to aid bridging of the latex spheres at the interface. The spheres retained their shape when alcohol was added to disrupt the oil/water interface.

Dinsmore et al. named permeable capsules obtained by self-assembly of colloidal particles colloidosomes [START_REF] Dinsmore | Colloidosomes: Selectively permeable capsules composed of colloidal particles[END_REF]. Polystyrene particles self-assembled at an oil/water interface and were held in place via sintering and use of a binding agent, poly-L-lysine (fig. 1.22). Once completed, the colloidosomes were transferred to a continuous phase that was the same as the liquid they contained.

Sintering is only one method to ensure particle bridging. Alcohol groups on the surfaces of latex or silica particles have been cross-linked with polymers containing isocyanate groups to form urethane bonds [START_REF] Thompson | Covalently cross-linked colloidosomes[END_REF] [START_REF] Fielding | Preparation of pickering emulsions and colloidosomes using either a glycerol-functionalised silica sol[END_REF]. Bon We have seen how Pickering emulsions and colloidosomes can be formed. Both involve solid particles at their surface, and indeed microcapsules can be formed via Pickering emulsions. In the previous section, we saw the interest of using plasmonic resonance to build optical sensors. Gold particles exhibit this resonance, and the same particles can also be used to make Pickering emulsions. Combining all these possibilities, COMPASS developed its own sensor, which we will see next.

Plasmon-based mechanochromic microcapsules

Introduction A sensor of choice should be easy to implement, sensitive, yet at the same time provide an output that is readily observable. Mechanochromic polymers that can react to local changes in strain or pH have been developed [17]. Colloidosomes based on Pickering emulsions are stable and their physical and chemical properties can be modified. Pickering emulsions can be made with gold nanoparticles, and this metal exhibits plasmonic resonance which is well documented.

Combining these properties, COMPASS has made plasmonic-based mechanochromic microcapsules. AuNPs stabilize the oil-in-water Pickering emulsion. At the interface of the emulsion, a polymeric shell forms, and the AuNPs are embedded in this shell. Changes in local pH or stress affect the microcapsules' shape and thus the plasmonic resonance of their AuNPs (fig. 1.27). Different types of monomers were tested to form the shell.

Gold-silica microcapsules

A toluene-in-water emulsion was prepared with polyethoxysiloxane (PEOS) dissolved in the toluene phase and AuNPs in the water phase [27]. Water hydrolyzed PEOS, turning its alkoxysilane groups into silanols. Hydrolyzed PEOS could then either react with other hydrolyzed PEOS (alcohol condensation) or non-hydrolyzed PEOS (water condensation) to create a silica shell (fig. 1 As mentioned in section 1.4.1, charges influence the wetting properties of NPs and thus the stability of the Pickering emulsions they create. To verify that surface charges indeed control AuNP adsorption, one synthesis of gold-silica (Au-Si) microcapsules was performed at pH>4, and another was performed at pH<4 [27]. At pH>4 the emulsion droplets were transparent, which suggested colorless silica capsules with no or few AuNPs embedded in the surface (fig. 1.24). On the other hand, microcapsules synthesized at lower pH (<4) resulted in red microcapsules which became blue with increasing adsorption of AuNPs at the surface (fig. 1.25).

PVA films containing gold-silica microcapsules

To test the gold-silica microcapsules, they were subjected to mechanical strain. One could either manipulate individual microcapsules or embed them in a film, and conduct strain tests on the film.

A solution containing Au-Si microcapsules was added to polyvinyl alcohol (PVA) to create polymer films. The microcapsules retained their color and shape after their introduction in the films (fig. 1.26). The PVA films were subjected to a series of mechanical tests to assess their mechanochromic properties. Impact tests A weight was dropped from different heights on a PVA film doped with Au-Si microcapsules. For different kinetic energies at impact, the peak plasmonic resonance changed (1.28).

Figure 1.28 -A weight was dropped from different heights onto a PVA film containing Au-Si microcapsules. Plasmonic peak resonance wavelengths were plotted with respect to kinetic energy at impact. Inset: extinction spectra for different impact energies, normalized at 420 nm. Scale bars 2 mm. [27] Perforation tests A hole puncher was used to perforate the PVA film. Microcapsules near the hole were strongly deformed and changed color whereas those farther away retained their blue color and spherical shape (fig. 1

.29).

A color gradient was formed and one could easily localize damage in the film. It also showed that only microcapsules near the deformation changed color, so we did not have "false positives." The Au-Si microcapsules changed color with strain but they were found to be quite brittle (fig. 1.30). This made them unlikely to be reusable, and the color change was also unlikely to be reversible. The COMPASS group thus opted for a more elastic capsule, made with acrylates instead of silica. Stretch tests Similarly to Au-Si microcapsules, Au-acrylate microcapsules were also embedded in PVA film. The film was then subjected to uniaxial and biaxial strain (400% in the horizontal direction and 200% in the vertical direction), which showed that the microcapsules were elongated and changed color (fig. 1.32). After release of the stress, in both uniaxial and biaxial strains, the microcapsules regained their spherical shape and blue color as seen in fig. 1.32a. Qualitatively the Au-acrylate microcapsules function like their silica-based counterparts. However, quantitative measurements linking colors to strain have not been conducted yet. Also, in fig. 1.32c, we see some holes form in the microcapsule shell. The microcapsules could be improved.

Advantages and drawbacks

We have just seen the sensor developed by COMPASS. How does it compare to the technologies mentioned in table 1.1? Let us review.

Advantages Disadvantages Easy synthesis Sensitive Reversible color change Each microcapsule is an individual sensor

Gold is costly (Si-based) can be brittle Complex optomechanic coupling Table 1.2 -Pros and cons of the plasmonic microcapsules.

Conclusion

We have seen different technologies that could be used to create optical sensors. I have discussed the merits of using plasmonic resonance and Pickering emulsions as mechanochromic objects, and presented the gold-silica and gold-acrylate microcapsules developed by COMPASS. The microcapsules can be embedded in polymer films and change color according to local stress. Gold-acrylate microcapsules can change color according to pH. These properties could be useful to detect bacterial biofilms, for example, which exert forces on substrates and change the local pH [69][70]. Applications would include the hospital or the food industry.

There is still a need to further investigate the microcapsules. As seen earlier, toluene was used as the oil phase in the synthesis of the plasmonbased microcapsules. Toluene is toxic, so a safer but just as or more efficient alternative should be pursued. Films could be embedded with pH-dependent microcapsules and their color change could be compared to that of microcapsules in solution.

The following chapters will discuss the preparation of AuNPs and the microcapsules that use them. The different syntheses and techniques used to evaluate the AuNPs and the microcapsules will be presented.

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Chapter 2

Gold nanoparticle synthesis

Introduction

This chapter focuses on the gold nanoparticles (AuNPs) used to create Pickering emulsions. Several synthesis conditions were explored to determine their effect on the AuNPs. It is important to find suitable syntheses which will give us the desired colors and stability in our microcapsules later on, becuase NP size and shape dictate the plasmonic resonance and the stability of the Pickering emulsions. Can we control these parameters? Can we tune them by modifying the synthesis?

Protocol

The protocol was based on Burel's method which itself is based on the synthesis proposed by Malassis et al [1]. A gold salt is reduced to gold by ascorbic acid. The gold is stabilized by a capping agent, PVP-DADMAN. The capping agent also prevents the aggregation of gold. Burel's method was adopted because of its ease of implementation compared to more conventional approaches such as "seed growth" synthesis. As we will see, the characteristics of the NPs (size, shape) can vary from synthesis to synthesis.

Comparison with other syntheses

In general, AuNPs can be obtained via the Turkevich method or a variant thereof. The Turkevich method reduces a gold salt (HAuCl 4 , gold (III) chloride hydrate) with sodium citrate [2]. Reaction conditions such as temperature and gold-to-citrate ratios affect the size distribution of the resulting AuNPs. Frens adapted the Turkevich method to obtain AuNPs ranging between 16 and 147 nm in size [3].

Brown et al. proposed a synthesis using hydroxylamine (NH 2 OH) as the reductant instead of citrate [4]. The advantages of NH 2 OH are twofold: syn-thesis can be performed at room temperature (citrate seeding requires boiling temperature) and the resulting AuNPs are more monodisperse. However the authors reported the presence of nanorods as well (fig. The synthesis developed by Malassis et al. [1] is interesting because it involves fewer steps than seed growth (one reduction instead of two) and it uses ascorbic acid as the reductant, which, compared to CTAB, has a weaker binding to the NPs. With this method the authors were able to obtain spherical AuNPs.

Synthesis of the AuNPs

Reagents HAuCl 4 (Alfa Aesar, USA) was used as the initial gold salt. Poly(diallyl dimethylammoniumnitrate-co-1)vinylpyrrolidone, or PVP-DADMAN (Solvay, France) was used as the capping agent. Ascorbic acid (Sigma Aldrich, USA) was used as the reducing agent.

PVP-DADMAN is a polymer with a backbone composed of PVP and DADMAN (fig. 2.3). There are 99 units of PVP per unit of DADMAN. PVP is amphiphilic, nonionic, and readily adsorbs on different metals, including gold [6]. The abundance of PVP units in PVP-DADMAN confers hydrophobicity to the backbone of the molecule. AuNPs capped with PVP-DADMAN can adsorb more easily at the interface between the aqueous and oil phases, and their contact angle with either phase will be closer to 90° [ 7][8].

N O N + NO -

Synthesis

A 100 mL round bottom flask and magnetic stir bar were cleaned with aqua regia. HAuCl 4 and PVP-DADMAN were added to 100 mL of ultrapure water in the round bottom flask to create a solution with the following concentrations: [HAuCl 4 ]=2.5x10 -3 M and [PVP-DADMAN]=5.7x10 -6 M. The magnetic stir bar was added and the round bottom flask was placed in an aluminum heat block. A condenser was placed on top of the round bottom flask. The heat block was set to 97 °C and stirred at 240 rpm. Once boiling was observed in the round bottom flask, 5 mL of ascorbic acid solution in ultrapure water (0.1 M) was added to the system. Then aluminum foil was wrapped around the round bottom flask. The system was left to heat and stir for one hour (fig. 2.4).

Characterization of AuNPs

To determine the uniformity between different batches of AuNPs, characterization is necessary. This includes evaluating the concentration of the objects, as well as their size distribution and shape. These factors affect the adsorption rate of the AuNPs on the emulsion interface and the color of the microcapsules thus obtained. UV-vis spectrophotometry, TEM and SEM image analysis, and SAXS were used to characterize the AuNPs.

Determination of gold concentration in AuNP solution

A simple method to estimate gold concentration is UV-vis spectroscopy, since according to the Beer-Lambert law, absorbance is proportional to the concentration of the attenuating material (in this case, the AuNPs).

Hendel et al. used AuNPs of different sizes, with different stabilizers, and different reductants, and compared the UV-vis spectra at 400 nm [9]. The absorbance at 400 nm, Abs 400 , is a useful benchmark for several reasons. First, below 400 nm, absorbance is increasingly influenced by organic substances, whereas above 400 nm, it is more and more influenced by localized surface plasmon resonance. Second, the photon energy at 400 nm is in the range of interband transition energies in bulk gold from 5d to 6sp. Therefore, Abs 400 is not affected NP size, shape, surface chemistry, etc. and Au(0) concentration should be linearly proportional to Abs 400 [10]. This is why I will present UV-vis spectra normalized at 400 nm. The results from Hendel et al. could be used as a yardstick for the AuNPs of this thesis, but the data are still not a one-size-fits-all solution. Indeed, the authors reported an uncertainty of [Au(0)] up to 30%. I therefore performed my own measurements, specific to my system.

A sample of AuNP solution was weighed, then placed in an oven to evaporate. After complete evaporation it was weighed again. All of the HAuCl 4 was assumed to have reduced to gold during synthesis. Knowing the initial volume of the solution and the difference in weight before and after evaporation, the volumic concentration and molar concentration of gold in the solution were estimated. UV-vis spectra of several dilutions of the sample were used to plot a calibration curve for future reference (fig.

2.5).

Image analysis

AuNPs obtained using the protocol described in 2.2.2 were observed in TEM and in SEM at the Singh Center of Nano of the University of Pennsylvania using a JSM-7500F (JEOL, Japan). The images yielded information on the size and shapes of the AuNPs (fig. 2.6). The observed NPs were a mix of spherical and polyhedral objects. Size dispersity was estimated to be 34%. Average diameter of the objects was 39 nm.

SAXS measurements

Techniques such as TEM can provide information about the size of the NPs. However, TEM requires a skilled operator and a number of lengthy preparation steps. Operator biases can occur and images may not represent the system as a whole [11]. More importantly, TEM is not suitable for in situ measurements [12]. X-ray scattering could be used as a complementary analysis method [13]. The size range of study in small angle X ray scattering (SAXS) is on the order of tens of nanometers, i.e. the size range of the AuNPs, and gold scatters well.

NP solutions

Protocol A solution of AuNPs prepared as described in section 2.2.2 was diluted several times in ultrapure water (non diluted, 1:2, 1:4, and 1:8 dilutions). Each dilution and a blank (ultrapure water) were measured with a Xeuss 2.0 SAXS/WAXS beam line (Xenocs, France) at the LRSM of the University of Pennsylvania (Philadelphia, PA, USA). The intensities of the samples were subtracted by that of the blank. Normalization with respect to the concentration showed that the dilutions were properly prepared and that the SAXS setup worked as expected (fig. 2.7). Then, the signals were fitted using either a normal distribution (eqn 2.1) or lognormal distribution (eqn 2.2) fit to give an estimate of the sizes of the AuNPs (fig. 2.8), assuming they were all spherical.

f µ,σ (q) = 1 √ 2π e - (q -µ) 2 2σ 2 (2.1) f µ,σ (q) = 1 √ 2πq e - (ln(q) -µ) 2 2σ 2 (2.2)
where

    
µ is the average σ is the standard deviation q is the scattering vector These fits can help estimate the scattered intensity I s , such that

I s = I 0 x f µ,σ (x)P m (x) (2.3)
where P m is the form factor. Results Fig. 2.8 shows that the signals are proportional to the dilution factors and that the mean radii and standard deviations were similar between dilutions, for either normal or lognormal fits. The size distribution values found with SAXS differed slightly from those obtained via TEM image analysis (fig. 2.9). The difference may be attributed to how particles were counted. If the NPs were too close to one another they could have been difficult to differentiate individually. Also, smoothing out the edges (to obtain rounder objects for easier size measurement) could have resulted in objects with measured sizes slightly varying from their real sizes. The reader is also reminded that the AuNPs were all assumed to be spherical during fitting of the SAXS data. Compared to TEM, the size differences were not that significant after all.

Changes in synthesis conditions

As mentioned earlier, the appearance of nanoparticles was affected by their preparation. Several synthesis conditions were modified to see if the resulting AuNPs would differ from those made using the original method.

Effect of capping agent concentration

PVP-DADMAN stabilizes the AuNPs and prevents them from aggregating. Changing the concentration of the capping agent could affect the size and quality of the AuNPs (original protocol: [PVP-DADMAN]=5.7x10 -6 M). Several batches were prepared with different PVP-DADMAN concentrations: 5.7x10 -7 M, 2.85x10 -6 M, 5.7x10 -6 M (fig. 2.10). UV-vis spectra do not seem to show a monotonic relationship between [PVP-DADMAN] and extinction intensity or wavelength. It is still worth noting that the original concentration of PVP-DADMAN yielded the highest extinction intensity and is certainly the optimal concentration for this synthesis. The results show that changing the capping agent concentration does not significantly change the resonance peak positions. There is an optimal concentration, but it doesn't give a much higher or narrower peak than the others. AuNP stability seems to be independent on the capping agent. Still, the use of ascorbic acid is also a source of charges: what is its effect on the NPs?

Effect of reductant concentration

The concentration in ascorbic acid was modified. The acid brings negative charges to the AuNPs and affects their wettability properties, and thus their adsorption on the droplets/microcapsules where they will be used.

The original protocol's 0.1 M ascorbic acid solution was compared to two separate samples prepared with different ascorbic acid concentrations, 2.5x10 -2 M, and 5x10 -2 M. The resulting UV-vis spectra suggest a narrowing of the peaks with increasing ascorbic acid concentration as well a possible increase in AuNP size, probably due to aggregation (fig. 2.11). Ascorbic acid concentration has more of an effect on the resonance peak position and bandwidth than PVP-DADMAN concentration.

These results are consistent with the work presented from Malassis et al. [1], where the authors showed that ascorbic acid had a dual role as reductant and stabilizer. In that same article the authors were also able to tune the sizes of the NPs by altering pH of either the gold salt solution or the ascorbic acid (fig. 2.12), which is probably why we also see a shift in peak positions with respect to ascorbic acid concentration. It was hypothesized that at higher pH, reactivity between the gold salt and ascorbic acid increased, leading ro the formation of more nuclei, hence the smaller NPs.

Effect of synthesis time

The original protocol has a run time of 60 minutes. A sufficient amount of time is expected in order to properly generate monodisperse and stable AuNPs. That said, if shorter syntheses yield comparable AuNPs, we could significantly save time.

A synthesis was started following the same conditions as given in 2.2.2. Immediately after addition of ascorbic acid solution, a sample was taken from the system. Then the condenser was fastened again to continue the process. This was repeated every 10 minutes until 60 minutes (fig. 2.13).

The results do not show a significant difference between synthesis times. This implies that reduction by ascorbic acid happens instantaneously in the system (and indeed the solution changes color instantly once the reductant is introduced), but a prolonged experiment would ensure that all the gold salt is reduced.

Scalability

The AuNP synthesis is usually performed in a 100 mL round bottom flask. As seen earlier, a single 100 mL batch takes 1 hour to run, then is left overnight to let any oversized NPs to precipitate. If the AuNPs need to be concentrated, then the batch is centrifuged. Thus, unless several batches are prepared in parallel, AuNP synthesis can become time-consuming. For practical reasons it is therefore desirable to scale up AuNP synthesis.

The 100 mL synthesis was performed in an aluminum heating mantle of the same size. 500 mL versions of the glassware and the mantle were used to perform a synthesis five times the normal volume. However mounting a condenser on the 500 mL round bottom flask caused the solution to overflow. 25 mL was removed from the round bottom flask and the remaining 475 mL was used for the synthesis.

The 100 mL and 475 mL batches had differenct UV-vis peak positions, which suggests that scaling up was unsuccessful (fig. 2.14). The differences probably came from the uneven heating (more exposure to air for the 500 mL flask) or mixing.

A simple visual inspection was sufficient to tell that the AuNP solution was different as well (fig.

Conclusion

AuNPs were synthesized according to a quicker and easier alternative to the traditional seed growth method. The NPs were characterized using TEM and SAXS.

UV-vis spectra of NP solutions suggest that changing experimental conditions could lead to a change in concentration and size of the AuNPs. Reaction time however did not significantly alter the result; the synthesis was later adapted to a shorter 40 minutes.

A calibration curve of the AuNP solution was constructed as a reference point for future syntheses. It should be stressed that a single curve should not be used for different synthesis conditions.

Using such information, one could modify the synthesis to their own needs. Since NP size affects plasmonic resonance, it would be possible to fine-tune the synthesis to obtain a desired resonance wavelength.

Characterization of the AuNPs could be explored further. In addition to SAXS, SANS (small-angle neutron scattering) could also provide useful information on the AuNPs, especially on the coverage of PVP-DADMAN. Using both SAXS and SANS could yield information on the inorganic and organic parts of the AuNPs: gold covered in thiols have been investigated in this way [11]. Also, we have seen that the current synthesis is not quite scalable yet: this presents a challenge for chemical engineering.

Speaking of scales, millifluidic or microfluidic syntheses have been developed as well [14] [15]. These could be useful alternatives, as they can yield controlled objects; I will soon discuss the merits of microfluidics as an experimental tool.

Chapter 3

Gold-acrylate microcapsule synthesis

Introduction

This chapter describes the synthesis of gold-acrylate microcapsules (Auacrylate MCs). The MCs contain AuNPs obtained with the protocol given in the previous chapter. Polymerization at the interface of the emulsion droplets will give us solid, elastic capsules with NPs to provide a plasmonic response to pH.

Experiments with Au-acrylate MCs were done to mainly address three questions: can we produce MCs in a controlled manner? Can we tune the conditions of synthesis to produce different sizes? Can we generate these MCs using a greener chemistry?

That last question is especially important for COMPASS and its industrial partner Solvay. As mentioned in previous chapters, the existing Auacrylate MCs were made with toluene-in-water Pickering emulsions. Toluene is well-known for its toxicity [1]. The initiator, AIBN, poses safety risks too [2]. If we can find a more environmentally friendly route to generate MCs, we can imagine more uses for them (e.g. medical and food industries).

Synthesis of MCs and their color-changing properties will be described. We will also see how different synthesis conditions affect the outcome.

Creating plasmonic polymers 3.2.1 Suspension polymerization

Monomers in the oil phase undergo suspension polymerization. This is a process in which monomers, insoluble in water, are dispersed and stabilized during polymerization, to produce a dispersed solid phase [3]. Stabilizers help reduce coalescence between monomer droplets and adhesion between polymer particles.

The monomers are in the oil phase. Yet polymerization occurs not in the bulk, but rather at the oil-water interface of the emulsion droplets [4]. Several hypotheses have been proposed to explain this phenomenon. Polarity differences between the oil phase and the polymer would cause the latter to migrate towards the interface [4]. Surface tension could also be a factor. The initial water/oil interface is split into two: water/polymer and oil/polymer. If the sum of the interfacial tensions of these new interfaces is lower than that of the initial one, the formation of hollow capsules is favored [5].

pH-based changes in plasmonic resonance

Plasmonic resonance depends on the distance between neighboring NPs, and Au-acrylate MCs can swell or shrink according to pH.

The polymer shell has acrylic acids which are known for their swelling properties [6]. If the pH is higher than the pK a of the carboxylic acid groups, the acids are deprotonated into carboxylates. Neighboring carboxylates will then repel one another because their charges are of the same polarity. Assuming a base is added, e.g. NaOH, free water molecules will surround the counterion and hydrate it. The introduction of the counterion and water in the polymer results in the swelling of the polymer matrix, and an increase in the distance between two neighboring AuNPs (fig. 3.1).

Changing synthesis conditions

I will discuss the changes made to the synthesis and present my results. Do emulsion sizes and appearance depend on the different settings? How can we control the process?

Reproducing previous experiments

Au-acrylate MCs were first synthesized according to Burel's original synthesis as described in her PhD thesis (section 3.5.2) to check the reproducibility of the process. Two phases were prepared (table 3 The two phases were added in a scintillation vial. The vial was placed in an ultrasonic bath at 30 °C for 15 min. HCl lowered the pH, which weakened the charges of the AuNPs, enhancing their adsorption at the interface, and promoted the formation of a Pickering emulsion. Once taken out of the bath, a sample of the emulsion was observed with optical microscopy. The remaining emulsion was then heated at 60 °C for one hour. This step activated the initiator AIBN and polymerization took place. Another sample was collected for observation, with the rest left to heat for another hour to complete the polymerization step. The most notable observation during the synthesis is the transition from pink droplets right after sonication to blue droplets/capsules after the heating step. As the AuNPs adsorbed on the interface more and more densely, the color of the capsules shifted. The polymerization could have also brought NPs closer to each other at the interface and caused a local change in refractive index.

New oil phase

Properties of Neobee

As mentioned earlier, we are looking for a substitute for toluene as the oil phase. Ideally this would be non toxic and non polluting. Neobee was tested for its potential as the oil phase.

Neobee (also sold as Miglyol) is the commercial name of a mix of capric and caprylic triglycerides (Stepan, USA), derived from coconut and palm kernel oils. Neobee is used in cosmetics and in the food industry. It has been tested as an oral delivery system for drugs [8]. As it is safe to use, it will open up new applications for the Au-acrylate MCs.

Before one can substitute toluene with Neobee as the oil phase, the physical and chemical properties of the two oils should be compared to predict if indeed Neobee would be a good candidate (table 3

Comparing toluene-and Neobee-based emulsions

Two samples were prepared under the same experimental conditions except for the oil phase: one was toluene-based and the other was Neobeebased. A common source of AuNP solution was used in each sample. No initiator was used, in case a given initiator interacted differently with a given oil phase, and because initiators were not necessary to induce polymerization. The NP solution was used without changing its original pH (2.9).

Image analysis was performed on pictures taken with optical microscopy. This consisted of taking a number of pictures and defining the droplets to measure their radii. With a large number of droplets, one could then evaluate their size distribution.

The results suggest that using Neobee gives smaller and more monodisperse droplets (fig. 3.4). pH-dependent color change and capsule stability were observed in the Neobee-based sample. Neobee is indeed a good substitute for toluene.

Changing the initiator

Instead of oil-soluble AIBN, a water-soluble initiator, KPS (potassium persulfate), was tested to compare Au-acrylate MCs obtained using different types of initiators. Both Neobee and toluene were tested as oil phases. One sample with AIBN in the oil phase, and another sample with KPS in the aqueous phase, were prepared with the same monomer-to-initiator ratios.

In the future, encapsulating a product such as a dye inside the MCs could be imagined. If the cargo is in contact with an oil-soluble initiator such as AIBN, this could result in unwanted reactions: this is another reason why KPS was tested.

In toluene KPS yielded more polydisperse droplets than AIBN (fig. 3.5). Also, the average radius was bigger when KPS was used.

In Neobee KPS yielded larger droplets. The overall sample (capsules + surrounding liquid) seemed to be slightly turbid compared to the preparation using AIBN (fig. 3.6) with small bubbles seeming to form on the capsules' surface.

Discussion

For both oil phases, we see that KPS gave slightly larger capsules. The difference in size could be due to a difference in polymerization rate. However, it should be noted no significant size difference was observed between the droplets right after sonication, and the capsules after 2 hours of polymerization. Indeed the different sizes between the the AIBN-and KPS-initiated samples were already observed even before the polymerization step.

This observation suggests that 1. Polymerization does not change droplet/capsule size or the change is negligible.

2. The size differences seen between the two samples (AIBN vs. KPS) do not depend on the rate of polymerization.

Thus, the images do not show the degree of polymerization. Comparing the monomers and the capsules in NMR could give more information on this, and cross sections of the capsules in SEM could reveal the thickness of the polymer shells.

If droplet size differences were seen before polymerization, perhaps each initiator changed their solvents' viscosity and thus the response to shear during sonication. Colors did not seem to vary: all capsules were blue. Colorimetry analysis would confirm this in a more quantitative fashion, but we can confidently say the capsules' surfaces were packed with NPs. Plus, they all exhibited the expected pH-dependent change in color, so they were all functional.

Summary: effect of oil phases and initiators on capsules

For more clarity, let us review the box chart data presented so far in a The table 3.3 indicates that the median radii of the capsules were consistently larger in toluene, whatever the initiator used. In table 3.2 we saw that the viscosity at 25 °C of toluene was approximately twice that of Neobee. Interestingly, this is about the same difference between the capsule radii when we compare the two oil phases. Due to its higher viscosity, toluene would require stronger shearing to obtain smaller emulsion droplets. To show an empirical relationship between viscosity and droplet size, we would need more data, but this could help estimate and control the sizes of the MCs. This could be an avenue to investigate in the future.

It also seems that using KPS led to larger droplets, for either oil phase. As stated earlier, droplets right after sonication did not change in size during the polymerization step, and the size difference between AIBN-and KPSbased samples could be due to physical differences rather than chemical, e.g. the solvents' viscosities. To confirm this, adding different amounts of initiator in its solvent (KPS in water, AIBN in oil) and measuring the viscosity could be done.

The initiator-less tests (fig. 3.4) and the ones with initiator resulted in different colored MCs. This was likely due to the difference in pH: the samples sans initiator were ran at the initial pH of the NP solution (pH=2.9). The other samples were tested at a lower pH of approximately 1 and had HCl solution added to the NP solution prior to emulsification. This brings us to the next section: what other effects do pH have on the capsules?

Changing pH

The original protocol requires a very low pH, close to 1. This is achieved by adding HCl in the aqueous phase. A low pH value is chosen to promote adsorption of AuNPs at the droplet interface. The threshold pH is 4: below this value, the charges on the AuNPs are weakened, and electrostatic repulsion is also diminished, allowing the AuNPs to adsorb more easily to the water/oil interface (fig. 3.7).

If the pH is too acidic, potential applications of our mechanochromic MCs could be quite limited. Also, the initiator may not perform as well if the pH is too low (despite pH seemingly a nonfactor [9]). Can MCs still be formed at higher pH?

KPS

It appears that MC size distribution does not depend on pH (fig. 3.8). In terms of color however, the lowest pH only gave blue droplets, and the two others resulted in rather red or mauve-colored MCs. This did not prevent them from changing to an intense red once a basic solution (pH 13) was introduced.

This implies that there is no visible effect of pH on droplet size, but most likely on the adsorption of the NPs at the droplet interface.

AIBN

In the case of AIBN, MC size distribution did not vary much between pH 3.0 and 4.2 but at pH 1.2 the capsules were noticeably smaller 3.10). Colors were different between pH, going from blue (pH 1.2) to a mix of blue and purple, (pH 3.0) to red and purple (pH 4.2). Once again, color change was observed when a base was added, but the redder capsules at pH 4.2 did not show a significant change in color, since they were already red.

These results also show the colors' dependency on pH. However, the capsules obtained at pH 1.2 were much smaller than those generated at higher pH. Is this result an outlier? To dispel any doubts the same experiment should be redone. But this illustrates an issue with the sonication method of emulsification: reproducibility.

Discussion

Both initiators performed well in the pH range studied here, 1-4, since stable MCs exhibiting pH-dependent color changes were obtained.

pH doubly affects capsule color if we recall section 3.2:

1. At lower pH, repulsion between AuNPs is weakened and more can adsorb on the droplet interface.

2. At higher pH, swelling of the polymer stretches the interparticle length between neighboring AuNPs on the MC surface.

Here, we are not dealing with swelling because we are always below the pKa of acrylic acid (4.25 at 25 °C [10]) so any color difference is due to a difference in adsorption: more NPs adsorbed would mean less of a distance between them, and more blue capsules.

The results highlight that pH is important to obtaining and controlling the colors of the capsules. Since adsorption is more or less simultaneous to polymerization, once the polymer is formed, additional NPs are unlikely to adsorb. To promote adsorption of NPs, the operational pH should be kept very low. Once the NPs are in place, then polymerization can be done at the emulsion interface.

Summary: synthesis conditions

We have seen the various effects of changing synthesis conditions on the resultant MCs. Key observations are:

-Using Neobee yields smaller droplets than toluene -pH affects the adsorption of the NPs Yet at the microscopic scale it is difficult to study interactions between neighboring NPs at the interface, and we can only infer that the interparticle distance is altered during the swelling and shrinking of the MCs by the overall color. Blue MCs suggest tightly packed NPs, but what is the reality? We would need to zoom in on the system, beyond optical microscopy. We revisit a technique presented in the previous chapter: SAXS.

SAXS experiments

Interactions at the microscopic scale can be studied with optical microscopy, but to better understand interactions at the nanoscopic scale, SAXS could be quite useful. The technique might be able to show what happens at the capsules' surface.

We read in the previous chapter that in SAXS, the measured intensity is proportional to the form factor P (q). Specifically we can say that it is proportional to the product P (q) * S(q) where S(q) is the structure factor. If the form factor gives information on the objects in a system themselves: size, shape, composition, etc., then the structure factor gives information on the objects' spatial arrangement to one another. To express the structure factor, then, we can divide the scattered intensity I(q) by the form factor P (q). Since all the objects are considered spherical, w the form factor value can be approximated as 1, which means the structure factor is directly observed in the plot (fig. As the color change of the MCs is reversible, so are q-values of the observed inflection points in the signals. While these inflection points are not pronounced, likely due to the polydispersity of the sample, the q-values can be translated into lengths. Could these lengths represent the distance between two neighboring AuNPs? This could be the start of an in-depth study of Au-acrylate MCs.

Conclusion

Au-acrylate MCs were generated with different conditions: nature of the oil phase, type of initiator, sonication conditions, pH. To summarize the key observations:

-Neobee works just as well as toluene as an oil phase, yielding plasmonic MCs. Sizes of the droplets were smaller when using Neobee; this is likely due to Neobee's lower viscosity. Plus, using Neobee gave less polydispersity. -Water-soluble KPS can be used in lieu of oil-soluble AIBN. Slightly larger droplets were observed with the former as the initiator.

-pH seems to affect the color of the droplets. Even between pH 1 and 3 the differences are noticeable. -SAXS may shed more light on the nanoscopic interactions occuring at the MC surface. Neobee is a safe substitute for toluene, and opens up more possible applications for the MCs. We can imagine using them in polymer films to detect bacterial biofilms because when these are created, they acidify their immediate environment [11]. Toluene would kill bacteria anyway, so it would not be a good choice to on which to build an optical sensor.

Taking these observations into account, we can change synthesis conditions to our needs. Possible future experiments could include a more indepth study in SAXS, evaluation of the conversion rates of monomers, changing the quantities of initiator, changing the temperature, and real-time observation of the transition from red to blue during the heating/polymerization step.

MCs were successfully produced, but the MCs were rather polydisperse. Reproducibility is also an issue as shown in the pH experiments with AIBN (section 3.3.5). In the next chapter I will propose a new method of emulsification that could provide more control of the process: microfluidics.

Fluid mechanics at the microscale

The claims made earlier about control and fine-tuning in microfluidics can be justified with the physics at small scales. To illustrate how fluids behave at smaller dimensions, let us introduce the Reynolds number. This dimensionless number is the ratio between intertial to viscous forces (eq. 4.1) [4].

Re = ρvL µ (4.1)
where

        
ρ is the density of the fluid v is the velocity L is the characteristic length µ is the dynamic viscosity As L gets smaller and smaller, so will the Reynolds number. When Re <2000, a laminar flow regime appears, as opposed to a turbulent flow regime (Re >4000). In a laminar flow, only diffusion matters (no convective mixing), so the behavior is very predictable. Microfluidic systems are almost always in the laminar flow regime. Based on the dimensions of the chip and flow rates used throughout this thesis, here, Re ∼10.

Another important dimensionless number is the Peclet number, which is the ratio between advection and diffusion rates (eq. 4.2) [5].

P e = vL D (4.2)
where D is the diffusion coefficient. Once again, for small values of L, the Peclet number will also be small, which means diffusion is dominant.

Reduced dimensions also imply capillary forces dominating gravitational forces (low Bond number).

Over three decades [1], these interesting properties have been used to create a variety of microfluidic devices, such as minireactors [6], cell sorters [7], and emulsion generators [8], as we will see shortly.

(Pickering) emulsions in microfluidics

There are many ways to create emulsions in microfluidics. One possibility is the use of flow-focusing devices [8]. These operate by bringing two phases together at a cross-shaped junction [9]. In the middle, a dispersed phase is introduced. A continuous phase arrives from either side of the middle channel, and pinches off the dispersed phase: this results in the formation of an emulsion (dispersed phase-in-continuous phase). The nature of the emulsion could be oil-in-water (o/w) as in our case, or water-in-oil (w/o) [10], or even double or triple emulsions (w/o/w for example, fig. (d-f) Microscopy photographs of smaller double emulsions at high flow rates with single, double, and triple cores, respectively. Scale bar 300 µm. [11] Usually surfactants are the stabilizer of choice when making emulsions, but Pickering emulsions in microfluidics is relatively recent, around fifteen years old [13]. Priest et al. used flow-focusing to generate dodecane-in-water droplets [14] in glass channels (fig. 4.3). Silica NPs hydrophobized with hexadecylsilane were used as the stabilizer. The authors observed coalescence of the droplets, but also noted that coalescence was delayed with increased NP concentration: NP concentration plays an important role in the stability of droplets. Baret et al. showed the influence of the channel lengths on coalescence events [16]. Although their work was on a surfactant-stabilized system, it is applicable to Pickering emulsions as well. An increase in length gives more time for surfactants to pack at the interface, leading to stabilization of droplets with lower surfactant concentration (fig. 4.6). This dynamic process is mainly governed by diffusion of the surfactant micelles. The authors estimated a time scale for surfactant adsorption at the droplet interface on the 1-50 ms range, which was also on the range of the residence time of their droplets, i.e. the time the droplets spend in a chip after their formation. For comparison, the chips presented throughout this thesis are much longer than those used by Baret These examples illustrate the need to properly control various parameters to promote emulsion stability: flow rates, particle concentration, residence time. As shown by Schroder et al. [15], generating stable Pickering emulsions in microfluidics is synonymous to finding a balance between different physical parameters, and not just increasing or decreasing a single quantity (for example, distance between droplets or particle concentration). Guided by the literature and by colleagues at the LOF (Laboratory of the Future, Bordeaux, France) who specialize in microfluidics, I conducted experiments to find suitable conditions for generating stable o/w Pickering ). Increasing the length of the channel after production helps stabilize emulsions. [16] emulsions in PDMS (polydimethylsiloxane, fig. 4.9). More specifically, this meant preparing the chips for o/w systems and studying the effects of flow rates and AuNP concentration on the stability of the emulsion droplets.

Microfabrication of channels

Two steps are required before microfluidic experiments can be done: first, making the mold, then printing the chips.

Photolithography

PDMS chips are made using a silicon wafer as a master. Once a master is prepared, many chips can be reprinted with ease. The different steps are illustrated in fig. 4.7 First, a silicon wafer is spincoated with a photoresist polymer to a desired height. Then a mask is placed on top of the photoresist. Depending on the type of photoresist, the parts of the polymer exposed to UV will either be more soluble to a developer solvent (positive photoresist) or polymerize (negative photoresist).

In our case, SU-8 50, a negative photoresist, is used. Since polydimethylsiloxane (PDMS) is the chip material, the master surface is silanized. This allows for easier detachment of PDMS from the master.

For the masters of the chips used throughout this thesis, spin-coating is done for 7 s at 500 rpm, followed by 30 s at 2000 rpm, twice (2x 50 µm provides more level deposition than 1x 100 µm). UV exposure time is 6 seconds.

Printing PDMS chips

Once the master is finished, PDMS is prepared. Products such as Sylgard 184 (Dow, USA) are sold in two parts: PDMS and a curing agent. The ratio between the two can be chosen to give a desired stiffness, but for most purposes, the PDMS:curing agent weight ratio is 10:1. After mixing the two products of the PDMS kit, the mix is placed under vacuum to degas. The bubble-less PDMS is poured on the master and cured in an oven at 65 °C for an hour. After curing, the PDMS chip is cut out with a scalpel. Then the chip and a glass slide are placed in a plasma cleaner (fig. 4.8). Plasma cleans and leaves silanols on the surface of the PDMS and glass. The two surfaces can then form Si-O-Si bonds to create a seal. This is called plasma bonding.

Surface treatment of microchannels 4.3.1 PDMS: a practical polymer

Let us briefly review the advantages of using PDMS to form the microchip. A variety of materials have been used in microfluidics research (Si [17], glass [18]) but one of the most popular choices is PDMS (fig. 4.9) [19]. Compared to glass or silicon, it is more pliable, and more complex channel designs are feasible with PDMS in less time [20]. PDMS is also more economical than glass. PDMS does not require clean room conditions; it can be bonded to other materials in a number of ways [19]. Also thanks to its elastic properties, PDMS can also be fashioned into pumps and valves, e.g. the Quake valve [21], which opened up many new possibilities with the material. For these practical reasons, PDMS was selected as the material of the chips presented in this thesis.

Si CH

3 CH 3 O         n

On the utility of surface treatment

PDMS is a versatile material. But it is inherently hydrophobic, and the system here is an oil-in-water emulsion: to keep the dispersed phase in the middle of the channels, the PDMS walls must be hydrophilic. This can be achieved by surface functionalization of the PDMS prior to use. The possible bonds between PDMS and surface treatment agents are illustrated in fig. Once plasma-activated, the channels can be coated with different molecules. This is achieved by injecting a surface treatment agent into a freshly activated chip. One must act quickly because PDMS recovers its hydrophobicity over time [23]. As the hydrophilized PDMS surface is thermodynamically unstable, low molecular weight chains can diffuse from the bulk up to the surface within an hour of activation [24]. But what should be used to treat the surface, and how can its effectiveness be evaluated?

Experiments

Three treatments were tested, one per PDMS chip. All three chips were prepared with the same master and the same PDMS to curing agent ratio (10:1).

1. pure water 2. PVP-DADMAN 5.7 µM solution 3. PVP 5.7 µM solution Water was selected as a control. PVP-DADMAN is the capping agent of the AuNPs. Finally PVP was chosen as it is a well-documented PDMS surface treatment agent [22]. Estimated [Au 0 ]=0.06 M.

To evaluate the effectiveness of each surface treatment, colorimetry measurements were conducted in the chip. First, using a syringe pump, the surface treatment agent was injected in the chip for 60 seconds at a flow rate of 5 µL/min to fully wet the channels. Then, the syringe pump controlling the surface treatment agent was turned off, and another syringe pump controlling the AuNP solution was turned on for 60s at a flow rate of 5 µL/min. Each 60 s injection will be referred to as a wave. Surface treatment solution and AuNP solution were thus injected in alternating waves. Images were taken of the chip during these waves, then image analysis was performed to determine the intensity of light in the channels -green light, specifically, since the red AuNP solution would absorb more light in the green spectrum (∼ 550 nm). If more and more AuNPs adsorb on the PDMS walls, the intensity of green light would decrease. In other words, we are measuring transmittance over time.

Every 6 seconds, an image was taken of the chip. An area was selected in the channels and its intensity was recorded for each image. Meanwhile, a baseline was selected: an area in the chip outside the channels, since this part is never in contact with any fluid. Since ten images are produced per wave of treatment solution, an average light intensity per wave can be found and normalized by the baseline value (fig. 4.12).

Results

The closer the transmittance is to 1, the better the treatment is at preventing adsorption of AuNPs on the PDMS surface, and PVP is the best option out of the three in that regard, so it was chosen for preparing subsequent chips for the experiments. Interestingly, PVP-DADMAN performed the worst. That said, transmittance never went under .9 for all three treatments.

PVP-DADMAN is 99 parts of PVP for 1 part of DADMAN (fig. 2.3), so it is very similar to PVP. However, the PVP used in the experiment had a molecular weight of 360kg/mol, whereas PVP-DADMAN's is 60 kg/mol. As PVP's chains are much longer they will have more steric effect on the AuNPs, hindering their adsorption on the PDMS surface. Also, the DADMAN group of PVP-DADMAN may react with PDMS. DADMAC is quasi-identical to DADMAN, with Cl -instead of NO 3 -, and has been bonded to PDMS to create positively charged hydrogels [25]. The AuNPs here are slightly negatively charged due to ascorbic acid used during their synthesis [26]. The NPs are probably attracted to DADMAN groups that are on the surface of PDMS.

On adsorption kinetics

Transmittance seems to plateau around 250 s into the measurements: this could give information on the AuNP-PDMS adsorption kinetics. Since we are in a laminar flow regime (Re ∼ 10 with given settings, and considering AuNP solution's viscosity and density equal to water's), AuNP solution moves the slowest at the edges of the channels in a boundary layer, and the NPs at this region will likely adsorb on PDMS. How long would it take for AuNPs to diffuse to the PDMS surface? We are at low Reynolds number as well; we can then estimate the diffusion coefficient value using the Stokes-Einstein equation (eq. 4.3).

D = k B T 6πηR (4.3)
where

         k B is Boltzmann's constant v is the absolute temperature η is the viscosity of AuNP solution

R is the radius of the AuNPs

The PDMS channels are square; cross-section area is 100x100=10 4 µm 2 . η can be approximated to water's viscosity, 1 mPa.s, and k B T is approximated to 4x10 -21 J. This gives D ∼ 10 -11 m 2 s -1 . Now, we can estimate the times of diffusion and advection. Since flow rate is set at 5 µL/min, linear velocity v ∼ 10 -2 m/s. Diffusion time τ d ∼ R 2 /D ∼ 10 -5 s and advection time τ a ∼ R/v , and the ratio between the two gives vR/D, i.e. the Peclet number. P e ∼0.1<1 so the system is diffusion dominated.

Summary

Based on the colorimetry results, PVP was chosen as the surface treatment agent. Surface treatment will promote AuNP adsorption at the droplet interface, where they are needed to stabilize the emulsions, rather than the PDMS walls. In addition, the hydrophilization of PDMS makes it usable for o/w emulsions. These two factors will help us obtain stable emulsions.

As for the adsorption kinetics, the calculations suggest that AuNPs primarily in the boundary layer near the PDMS walls will adsorb to the channels, and the bulk of AuNP solution will not interact with the PDMS surface. Now, to ensure the generation of monodisperse droplets, we must also understand how different flow rates for both continuous and dispersed phases affect droplet size and stability. This is crucial because flow rates and ratios between flow rates will dictate the flow regime in the chip, as I will discuss shortly. [27] and are convected downstream by the continuous phase. In the jetting regime, the dispersed phase exits the injection source as a thread or jet, which destabilizes and forms droplets. The co-flow regime is when both phases flow concentrically in the same direction.

Formation of Pickering emulsion in microchannels

In a flow-focusing structure, two models have been proposed to explain the mechanisms of dripping and jetting. The first is the shearing model, where droplet size is related to the reciprocal of the capillary number [28]. The second model, the rate-of-flow-controlled breakup model, where the size of the droplet is only related to the ratio of the flow rates of the dispersed and continuous phases [29].

A map of flow rates can be drawn to show where the transitions between flow regimes happen in the chip. Using such a map, it is possible to predict which flow rate ratios are suitable for dripping, which is the desired regime.

Construction of a flow diagram

Chip design A schematic of the chip is shown in fig. 4.14. The dispersed phase is Neobee, and the continuous phase is AuNP solution. The two phases come together at the nozzle, where flow-focusing occurs. The generated droplets enter a serpentine structure to lengthen the transit time in the chip and thus the probability of AuNP adsorption at droplet interface. The droplets then exit via a wider channel named the coalescence chamber. This structure is wider than the rest of the channels which allows the droplets to slow down, and the experimenter to see any potential coalescence events. This rear-packing phenomenon is analogous to what has been observed in bubbles. Kotula and Anna generated bubbles in a silica particle solution [30]. The authors hypothesized the following to describe adsorption and its effects on bubble dynamics. First, a freshly generated bubble has no particles adsorbed on it (fig. 4.18, first row). Then as particles move towards the rear (second row) they act as a solid layer, changing the tangential stress condition at the bubble interface from zero stress at the clean air-water interface, to zero slip at the particle-covered interface, which increases the drag in the thin film near the rear of the bubble. The increased drag causes the rear of the bubble to travel at a slower velocity than the front, causing the front and rear of the bubble to separate and form a thin neck (second row). As particles continue to adsorb to the interface, the particle stabilized region increases in size, and the neck propagates forward (third row). Once the capillary pressure is able to overcome the difference in shear stress at the front and rear of the bubble, the neck expands until a more uniform slug shape is observed. The bubble velocity decreases due to added shear stress along the channel walls resulting from the adsorbed particle layer. The particles may increase the effective viscosity at the air-water interface, promoting a recirculation of the outer phase in the neck of the bubble, pushing the particles towards the rear. That said, the authors could not explain why particles do not continue to convect along the surface to form a close-packed layer at the front of the bubble. ). However, since the oil phase moved at the same speed as the droplet, the authors compared the system to Taylor bubbles rising in capillaries in a steady continuous phase [31]. Such bubbles' stagnation points (points where local fluid velocity is null) are located at the edges. Likewise, the droplets' stagnation points would have been at their edges, implying a small asymmetry in the continuous phase flow profile, promoting surfactant adsorption at the rear of the droplets. If we then accept that AuNPs primarily adsorb at the rear of the Neobee droplets, we are faced with geometrical constraints. Observation of droplets at different flow rates show that while the width of the droplets stays the same, the length varies. This implies the droplet volumes and surface areas change as well. However, since the AuNP adsorption is limited to the rear area (which does not vary), the same number of AuNPs will need to cover a larger surface area for longer droplets. If the surface area of the droplets is reduced, then AuNPs will be able to cover the droplets more densely, and increase stability of the Pickering emulsions.

Mapping key parameters

As just mentioned, due to the rear-packing of AuNPs at the droplet interface, to gain in NPs per surface area and thus in Pickering emulsion stability, surface area must be reduced. Also, increasing residence time in the chip will be useful since this allows more time for the AuNPs to adsorb on the droplets.

Here are the key parameters that could be either set or found via image analysis of the chip, which can guide us towards the ideal droplet: small, high generation frequency, round.

- Aspect ratio The sphere is the most stable shape energetically speaking and has the smallest surface area to volume ratio. The closer to 1, the better the aspect ratio. Increasing Q w and reducing Q o seems to be the way to reach this value (fig. 4.23). Since the channel widths are fixed, a decrease in volume (fig. 4.21 corresponds to a decrease in aspect ratio as well.

Finding a balance A certain flow rate pair may be good for one parameter but not for another. For instance, considering figs. 4.21 and 4.22, if small droplets are preferred, the flow rates should be increased. But doing so would decrease the residence time. There is a compromise to be made. If we take each parameter and find a common set of flow rates that is beneficial for some or all of the parameters (e.g. aspect ratio 1, low volume, and long residence time), we can find the "optimal" pair of flow rates to use in the chip. Furthermore, we might be able to conceive a dimensionless number that could directly tell us if we will obtain droplets that exhibit the desired properties and yield stable droplets.

So far we have seen the different steps taken to maximize our chances to get stable Pickering emulsions. To reiterate, what we want in our system are: and µ are the standard deviation and average, respectively, of a sample of droplets; a Gaussian fit of the droplet size distribution can be used to estimate these values. Image analysis was performed on chip-generated and sonicated emulsions to find their size distributions (fig. 4.26). PDI of the chip-generated droplets is 0.19, whereas PDI of sonicated droplets is 0.45. The high monodispersity found with microfluidics is a great improvement compared to sonicated emulsion droplets. This is positive news for us at COMPASS who want more control of our emulsions. The droplets are not very colored. We also know that the AuNPs are primarily located at the rear of the droplets. Maybe the droplet interface is not as packed with NPs as their sonicated counterparts. Then, how concen-trated should the NP solution be to ensure stability? Let us study a simple series of experiments.

Influence of NP concentration

As mentioned in the literature [15][16], coalescence can be prevented by increasing the concentration of the particles. An AuNP solution was diluted several times and images were taken in the coalescence zone for each dilution (fig. 4.27a). The differences are clear: diluted solutions lead to more coalescence, despite the surface treatment and tuning of flow rates. This is shown in the images as well as the PDI calculated from these images (fig.

Conclusion

This chapter introduced a new method of forming AuNP-stabilized Pickering emulsions, microfluidics, and presented the advantages of working at microscopic length scales. Since we want more control and monodispersity of the emulsions, microfluidics is indeed a good approach to achieve this. And as seen in fig. 4.26 the chip-based emulsification definitely gives more monodisperse objects compared to the sonication method. It should be noted that, due to the dimensions of the chip, the emulsions generated have a radius of approximately 50 µm, unlike the 20 µm seen in the sonicated emulsions.

We have also seen a curious phenomenon where AuNPs mostly adsorb at the rear of the droplets, and what was done to overcome this limitation: treating the surface, changing flow rates, measuring droplet sizes, etc., to find suitable conditions to generate droplets. The production of AuNPstabilized oil-in-water Pickering emulsions in microfluidics has not been attempted before. Thanks to the experiments described in this chapter we have a guide to choose good settings for stable Pickering emulsions.

However these droplets are not very colored, which implies a relatively low adsorption of AuNPs. The advantage of sonication is the energy it provides to the system. Chip geometry could be modified to generate smaller droplets or fitted with longer channels to increase residence time. Also it would be interesting to quantify coalescence events using an automated method. This is not trivial because we cannot track an individual droplet; droplets would leave the field of vision then return.

But now that know how to make stable Pickering emulsions, we can move on the next step: polymerization using microfluidics to create Au-acrylate microcapsules.

Chapter 5

Photopolymerization of Au-acrylate microcapsules in a microfluidic chip

Introduction

As we begin the final chapter, let us review what we have seen so far. First, the novel technology developed by COMPASS to build mechanochromic strain and pH sensors was presented: plasmonic microcapsules (MCs). Then, characterization of the gold nanoparticles (AuNPs) used in Pickering emulsions was described. Based on these emulsions, Au-acrylate MCs were fabricated. The differences observed for different synthesis conditions of the MCs were discussed as well. The MCs were shown to respond in a sensitive and reversible manner to pH. However, a couple problems arose during the synthesis of the MCs, namely polydispersity of the sizes, and lack of reproducibility.

To better control the system, an alternative approach to emulsification was taken. Instead of ultrasonication, microfluidic flow-focusing was selected to generate emulsions. The various parameters and experiments to build a suitable PDMS chip for stable, oil-in-water Pickering emulsions were described. Size distribution was narrower in chip-generated emulsion droplets compared to their sonicated counterpart.

Yet we are still one step removed from chip-generated Au-acrylate MCs. Acrylates need to be added and polymerized to obtain the final product. Photopolymerization was chosen instead of a thermally initiated process as used in the sonication-based syntheses. The usefulness of photopolymerization will be discussed and key observations from experiments using this technique will be reported in this chapter.

A need for a new method of polymerization

The original sonication method described in chapter 3 involved two 1h polymerization steps for a total of 2h at 65 °C. Two hours is too long to be practical, especially when the residence time of the droplets generated in the microfluidic chip is on the order of several minutes. While it is possible to collect droplets, then heat them to initiate polymerization, some coalescence still does occur once the emulsion droplets are removed from the chip, as seen in the previous chapter. If polymerization can be performed before any potential coalescence happens in or near the chip, handling of MCs will be easier since the polymers confer structure and elasticity to the MCs.

In short, we are looking for a fast-working polymerization that can be used in limited spaces such as a microfluidic chip. Photopolymerization is a suitable candidate. Let us see some examples illustrating the appeal of this particular type of polymerization.

Photopolymerization in microfluidics

Monomers or oligomers can be cured or photopolymerized under exposure to light [1]. A photoinitiator is used to convert photolytic energy into the reactive species, which can be a radical or a cation [2]. Usually highenergy, short-wavelength light such as UV is used to initiate the reaction but visible light is being more and more investigated as the light source as well [3]. Applications of photopolymerization include: coatings [4], microelectronics [5], and tissue engineering [6].

Advantages of photopolymerization compared to other modes of polymerization are high curing rates [7], high temporal resolution (ms or less [8]), lower operational temperatures [9], and less pollution [10] [11]. That last advantage is quite interesting for us at COMPASS who are shifting to greener chemistry, using Neobee instead of toluene as the oil phase for the emulsions, for example. A more recent trend is photopolymerization without photoinitiators or self-initiated polymerization [12][13] which further reduces the use of pollutants.

Photopolymerization has been proven to be a convenient method to obtain controlled polymers in microfludics since the Doyle group's findings using this particular technique [14]. Let us look at some strategies of microfluidic-based photopolymerization and the associated pros and cons. Polymerization can be done on-chip, after collection of emulsion droplets, or both on-chip and off-chip (fig. 5.1) [15]. The first approach of direct on-chip curing (fig. 5.1, top) allows for more control over coalescence events because the flow rates of the continuous and dispersed phases can be tuned to keep droplets separated. However, the reaction time must be shorter than the residence time of the droplets, otherwise they will exit the chip before being fully UV cured. To control the residence time, flow rates must be adjusted.

Reducing flow rates is not enough to buy more residence time, as droplet formation depends on flow rates too. Therefore, for a given droplet size, lengthening the chip could be necessary. Also, clogging can occur due to uncontrolled polymerization which results in polymers trapped in the microchannels. The second approach shown in fig. 5.1 (middle) is an off-chip polymerization. This method allows for more flexible curing times and prevents clogging. But, transfer from the chip to another container can cause coalescence of the droplets, leading to a loss of droplets and more polydisperse polymer capsules. The last suggestion (fig. 5.1, bottom) adds another UV curing step in case the first curing on-chip is insufficient. But the risk of clogging is still present. When setting up a UV curing system, the different advantages and drawbacks mentioned should be considered. We should also keep in mind that in the chip presented here (fig. 5.9), polymerization should not be performed in the thinner channels, because this will create plug-shaped polymers instead of spherical ones.

Figure 5.1 -Different approaches to photopolymerization in microfluidics. [15] Let us review some examples that show the merits of photopolymerization and what to expect in microfluidic systems similar to the one presented in this chapter.

Seiffert and Weitz [16] generated microgels of poly(N-isopropylacrylamide) (pNIPAAm) with on-chip UV curing. However they opted for polymer precursors instead of monomer solutions, because monomers present a couple of problems. First, there are inhomogeneities in the polymer network due to spatial concentration fluctuations during the early phase of the polymerization [17], and microphase separation during the sol-gel transition because of the heat created by polymerization [18]. Second, total degree of polymerization is not well-defined when monomers and crosslinkers are copolymerized. The polymer precursors were functionalized with dimethylaleimide (DMMI), which created dimers between polymer chains when exposed to UV (fig. 5.

2).

The authors also made microshells by making o/w/o emulsions (fig. 5.3D-F.). Comparison between the "normal" copolymerization of monomers and crosslinkers and the polymer-analogous photogelation showed that indeed no inhomogeneities were observed in the latter (fig. 5.4). Liu et al. generated quantum dot (QD)-encoded poly(ethylene glycol) diacrylate (PEGDA) microbeads [19]. The QDs were used to color-code the microbeads and build a barcode system based on colors. The authors overcame the on-chip photopolymerization's issues with a wide, serpentine structure which reduced coalescence and increased residence time during UV curing (fig. 5.5). The control of the sizes is worth noting here. By changing the flow rates, it was possible to obtain different-sized PEGDA microbeads in a monodisperse fashion each time (fig. 5.6). We have seen that controlled photopolymerization is feasible and does not require a lot of time or space. But there are some potential drawbacks to this approach. One is a possible lack of penetration of the UV photons (penetration depth on the order of hundreds of microns, e.g. in dextranmethacrylate [20] or poly(ethylene glycol) [21]), but this is perfectly fine for most microfluidic chips and small emulsions. Also, the UV transparency of PDMS makes it suitable for photopolymerization [8]. Challenges specific to photopolymerization in dispersed systems include scattering and absorption issues in turbid samples [8]. Since the continuous phase used in the previous and current chapters is concentrated AuNP solution, this will likely impact successful production of Au-acrylate MCs. If the refractive indices between Depending on location of UV curing vis-à-vis the chip, there is the risk of coalescence, clogged channels, or insufficient curing. Inhomogeneous crosslinking is also possible. More specific to the present case: AuNPs' scattering and absorption can affect the degree of polymerization.

No real precedent was found for the system of this chapter, i.e. photopolymerization applied on oil-in-water gold Pickering emulsions. While it is challenging to have no real point of reference, a successful synthesis of Auacrylate capsules would be a big step towards fabrication of new, exciting polymers in microfluidics. I will now discuss my findings.

Acrylate-in-water emulsions

The system has several components, as seen in table 5.1. To focus on the acrylates' polymerization, samples were tested without Neobee in the dispersed phase. The expected result would be polymer balls rather than polymer shells, but if successful it would still show that UV polymerization is feasible, despite the presence of gold.

Preliminary tests

Before using PDMS chips, the photoinitiator (PI) was tested off-chip. 2-hydroxy-2-methylpropiophenone (HMP, fig. 5.8), a free radical PI, was chosen, as its use with acrylates has been well documented [30] [31]. Three vials were prepared with butyl acrylate (BA), methyl methacrylate (MMA), and HMP. BA and MMA were mixed in equal weights (459 µL BA and 432 µL MMA), but the weight ratios of HMP to acrylates (BA and MMA combined) varied between vials: 0.3 wt%, 3 wt%, and 30 wt%. Each vial was exposed to UV (estimated intensity 0.9 W/cm 2 ) until hardening was observed. The UV lamp was placed approximately 3 cm away from the samples.

The 0.3 wt% mix became viscous after 10 minutes of exposure, but not solid. The vial became hot to the touch, likely due to a combined effect of the incident UV light and the exothermic polymerization process. The 3 wt% mix became solid after 5 minutes of exposure. The 30 wt% mix became viscous, but after 5 minutes the vial became too dangerously hot to continue further exposure to UV.

The different outcomes could be explained thus: too little PI is insufficient for polymerization, but too much creates a large number of short-chain polymers instead of a few long ones. The time scale of several minutes to polymerize could be attributed to a shallow depth of UV penetration. Out of the three tested, 3 wt% gave the best results and confirmed the proper functioning of HMP, and was chosen as the weight percentage for the dispersed phase in the microfluidic tests.

On-chip UV curing

Having confirmed that HMP did indeed respond to UV curing, the experiment environment was shifted to the microchip (schematic shown in fig.

5.9).

Setting up flow rates

According to the previous chapter, spherical droplets are desirable, since they have the smallest surface area per volume and the highest number of AuNPs per surface area. First, flow rates were adjusted to generate droplets of 100 µm or less in diameter in the coalescence zone (fig. 5.9e), given the height of the chip is 100 µm, and the width of the coalescence zone is 1000 µm. Once this size condition was met, then UV curing was performed. If droplets larger than 100 µm across are cured, the droplets could potentially polymerize as pancakes instead of spheres. Continuous phase (AuNP solution) flow rate Q w =40 µL/min and dispersed phase (Neobee, acrylate monomers and HMP) flow rate Q o =2 µL/min yielded adequate sizes (approximately 100 µm in diameter).

Protocol

Once adequate flow rates were found, UV light was shone near the outlet. This placement was to prevent unwanted polymerization and clogging upstream in the chip. Kapton tape was wrapped around the chip upstream of the illuminated region, as well as around the dispersed phase syringe. Once a steady supply of droplets was observed in the ROI, UV was applied for 3 min (time it took for droplets to traverse ROI and exit via tubing into a microcentrifuge tube for collection). Using a thermopile power sensor, intensity of the UV light was estimated at 1.3 W/cm 2 . The collected droplets were then washed in a PVP-DADMAN + ascorbic acid solution to match the conditions of the AuNP solution used as the continuous phase in the chip. Several centrifugation cycles (2000 rpm, 2 min) were performed to further wash the samples, which sedimented to the bottom of the microcentrifuge tube. This washing was to remove extra AuNP solution around the droplets/polymer balls, since AuNP solution presents two issues:

1. Gold masks the polymers in scanning electron microscopy (SEM).

2. The opaque AuNP solution makes observation of the polymers and their color difficult under optical microscopy (OM).

Results

Observation of chip After exposure to UV, no visible change was observed in the chip, such as clogging due to polymerization or degradation of the chip due to UV.

Optical microscopy After washing, droplets exhibited a slightly mauve color (fig. 5.10). If the wetting of the AuNPs at the acrylate/water interface is the same as at the Neobee/water interface, then this color is due to adsorption of the AuNPs. However we do not see the signature blue color seen in sonicated Au-acrylate capsules, which suggests a lower density of AuNPs on the surface. We also see some "lobes" or "patches" on the droplets. This could be due to inhomogeneous polymerization of the monomers.

Does gold interfere with polymerization?

In the beginning of this chapter, it was mentioned that gold could affect the quality of photopolymerized Au-acrylate MCs, because AuNP solution scatters and absorbs UV light. A transparent substitute for AuNP solution, SDS (concentration: 3 times CMC), was tested, even though this did not form a Pickering emulsion.

OM images show a concentrated population of polymerized capsules that stayed stable off-chip (fig. 5.12): UV curing was very successful. This highlights the potential problems caused by AuNP solution. But as AuNP is the main stabilizer of the emulsion, reducing its concentration will lead to more coalescence (cf section 4.5.2).

Conclusion

Droplets were likely successfully cured to give polymers, since centrifugation and washing did not destroy the objects collected from the chip post-UV exposure. However, judging by the overall aspect of the polymers, it appears that polymerization was uneven (fig. 5.10). Also, the color of the polymer balls suggest a rather low degree of AuNP adsorption. We have also seen that high AuNP concentration could negatively impact the degree of the polymerization.

Can the behavior of the AuNPs be altered to better stabilize the polymers? A simple way to change the system is to introduce Neobee in the dispersed phase. Not only will this make the system more comparable to the Au-acrylate capsules described in chapter 3, but hopefully the AuNPs have more affinity for Neobee than for the acrylates and will more readilty adsorb to the droplet interface. As for the uneven polymerization, adding Neobee could change the overall refractive index of the dispersed phase to reduce scattering.

Introducing Neobee

In section 5.3, the polymer "balls" obtained via photopolymerization had some issues with their appearance and their resistance to centrifugation and/or drying, as suggested by OM (fig. 5.10) and SEM (fig. 5.11). Possible differences between an oil phase sans Neobee and one with Neobee include optical properties (cf table 5.1) and wetting properties of the AuNPs.

In this section a couple of experiments will be described, both incorporating Neobee in the dispersed phase. First, a test in which the PI was added to the continuous phase, and second, assessment of gold's influence on polymerization.

Unless noted otherwise, the dispersed phase was prepared with the following volume ratio: HMP/BA/MMA/Neobee= 1:15.3:14.4:77.2.

Placing HMP in aqueous phase

The results presented so far had HMP in the oil phase along with the monomers. But what would happen if HMP was in the aqueous phase with the AuNPs instead?

HMP is known to be oil-soluble, but it is also water-soluble in a limited capacity. Water solubility of HMP at 20 °C is 13.3 g/L, and log P=1.62 at 25 °C [32]. If HMP is in the continuous phase, it will interact with monomers at the droplet interface and less with monomers in the bulk of the droplet, and maybe a more homogeneous polymer shell can be made.

To test this, HMP was added to the AuNP solution such that HMP was 1 wt% with respect to the acrylate monomers. UV intensity was estimated at 0.9 W/cm 2 , curing time was 90 s.

Results and discussion

A "sac" formed in the coalescence zone after UV curing. On occasion, incoming droplets would fuse with the sac. Increasing flow rates of either phase did not displace it. In addition to the sac, relatively smaller droplets (∼200-400 µm in diameter) also stuck to the channel walls, especially in the area directly illuminated by UV light. Incoming droplets ricocheted off the stuck ones. As UV light locally heated the chip, this could have led to an expansion of the droplets, causing them to stick to the PDMS walls.

The images illustrate a major challenge: removal of excess AuNPs.

Troubleshooting

We have seen how changing several settings can impact production of emulsion droplets and subsequent polymerization. One issue is the formation of a sac which clogs the chip and prevents collection of droplets since they fuse together. By using HMP in the dispersed phase, this issue was prevented.

There is also the problem with gold: on the one hand, it is required to stabilize the Pickering emulsions generated in the chip. On the other hand, it might cause scattering throughout chip (hence the "sac" issue and uneven polymerization) and makes imaging difficult in both OM and SEM. Centrifugation and washing of samples did not contribute enough to the removal of AuNPs.

An alternative approach is to first promote adsorption of AuNPs at the droplet interface, remove excess AuNPs, then perform photopolymerization.

Collection of droplets followed by photopolymerization

One of the concerns before the experiments were started was the scattering caused by gold and the subsequent attenuation of UV irradiation. But, as mentioned earlier, gold is needed to stabilize the Pickering emulsions. Separation of the different steps can both ensure AuNP adsorption and polymerization.

Protocol

Instead of on-chip polymerization, collection of the droplets, followed by UV curing off-chip was performed. In practice, the chip outlet was connected to PTFE tubing, which was placed in a large bottle containing PVP-DADMAN + ascorbic acid (same concentrations as in the AuNP solution). Then, UV light was shone on the bottle to polymerize the washed droplets.

On microcapsule colors

We are reminded that a high density of adsorbed AuNPs is linked to a blue-colored MC. MCs collected from the top of the bottle were more pinkish and slightly translucent (fig. 5.15). Some debris was showing behind the MCs. These are the remnants of MCs that burst shortly after they were placed on a glass slide for OM imaging. While the stability of the MCs was not perfect, the fact that they left behind colored patches, instead of diffusing into the surrounding medium, suggests that polymerization did occur, and that the polymer was colored by adsorbed AuNPs (fig. 5.16). Additional imaging was done in a capillary: this prevented bursting and gave a clearer view of the MCs (fig. 5.17). Judging by the dimensions of the capillary, the capsules are on the larger side (more than 100 µm across) but a magnifying effect of the capillary is possible. We also notice that some droplets are darker. If larger droplets are indeed darker, this could be the result of coalescence. If two droplets coalesce, the resulting surface area will be smaller than the sum of the initial droplets' surface area. Yet the total number of AuNPs is the sum of the AuNPs present on both initial droplets. Thus, in the coalesced droplet, there are more AuNPs per surface area and a darker color. However, instances of similarly sized capsules being pink or purple have been observed as well. If all the capsules went through the same chip under the same conditions, where do the color differences come from? Figure 5.17 -Capsules collected from the bottle after UV curing, in a capillary (outer diameter 1 mm). Image taken in transmission mode. Some final remarks on colors: they do not change when exposed to each bases or acids, unlike the Au-acrylate MCs obtained by sonication. This implies that swelling and shrinking of the MC shell are not present here. In addition, lowering the AuNP solution's pH prior to use in the chip did not promote adsorption either: a pH 2 sample had the same color as a pH 1 sample.

Conclusion

Photopolymerization was introduced as a practical alternative to thermally initiated polymerization. UV curing was used to obtain Au-acrylate MCs in microfluidics. Several challenges were encountered: polymerization was sometimes poorly controlled in the chip, and polymerized MCs were not stable for extended periods of time. Washing the droplets first, then UV curing them yielded rather stable MCs (larger ones over 300 µm across tend to burst when placed on a glass slide). Such a method allowed AuNPs to first adsorb on the droplets, and excess AuNPs to be removed prior to UV curing.

There are lingering questions: why do the AuNPs seem to stay on top of polymers as opposed to entering the polymer shell? Why are some MCs darker than others?

The process still needs to be improved to generate even stabler MCs, and more importantly, MCs that exhibit pH-or strain-dependent color change. This will probably require testing different UV intensities and maybe PIs other than HMP. It will take time but once such objects are successfully made, we can take full advantage of microfluidics to generate highly monodisperse MCs.

Conclusion and perspectives

We began this thesis by positing the need to quickly and easily assess wear and tear in polymers, due to their ubiquity in everyday life. We introduced mechanochromic stress sensors as a detection method. We saw several examples and their advantages and disadvantages. COMPASS made a new type of sensor based on Pickering emulsions and plasmonic resonance: mechanochromic microcapsules (MCs). As we learned more about these MCs, we also reviewed factors influencing plasmonic resonance and Pickering emulsion stability. The MCs were shown to change color in response to mechanical strain or pH, making them an interesting sensor to detect damage in materials or changes in pH (e.g. corrosion caused by chemical products or bacteria). This new technology was compared to the existing ones, and its advantages were presented.

In the second chapter I described the synthesis of gold nanoparticles (AuNPs).The synthesis was simpler than the traditional seed-growth method because it involved a single reduction step instead of two. It used ascorbic acid as the reductant, which is known to have weaker binding to the AuNPs than a more common reductant such as CTAB. This weaker binding facilitated ligand exhange, promoting functionalization of the AuNps with the capping agent developed from Solvay, PVP-DADMAN. Characterization of the AuNPs was done via UV-vis spectroscopy, SEM, and SAXS. On average the NPs were 30 nm in diameter, and the sizes found by measurement of SEM micrographs and by SAXS were in agreement, giving two complementary methods to estimate the sizes of the AuNPs. UV-vis spectra shapes did not depend much on synthesis time or concentration in capping agent, but the changing the quantity of ascorbic acid influenced the spectra, likely due to the pH-dependent reactivity between the gold salt and ascorbic acid. We also saw that scale-up (100 mL to 500 mL) was not successful.

In chapter three, the Au-acrylate MCs, using the AuNPs prepared in chapter two, were introduced. The principle of suspension polymerization to form the MCs and the pH-dependent color changes in the polymer shell were explained. Alternative methods and components were investigated to improve the safety of synthesis conditions of gold-acrylate MCs. For instance, the proof of concept had been done with toluene, a highly toxic substance, as the oil phase. Neobee was tested and proved to be a good substitute for toluene, yielding MCs that changed color reversibly to pH. Size distribution depended on the type of oil phase and initiator. Neobee gave smaller MCs than toluene. The differences could be attributed to Neobee's lower viscosity compared to toluene's. Water-soluble KPS gave larger MCs than oil-soluble AIBN, but as the droplets' sizes did not change during polymerization, the size differences between KPS-and AIBN-initiated samples were already present prior to polymerization. I therefore hypothesized that the differences were more physical than chemical: the addition of solutes in different phases would have changed their relative viscosities. pH affected the colors of the MCs, with the most blue MCs observed at pH 1, and red/purple MCs at pH around 3 to 4. Swelling of the MCs was ruled out because the acrylic acids found in the polymer shell had a pK a of 4.25. The colors of the MCs were thus due to the degree of AuNP adsorption, which decreased with increasing pH. We also noticed that polydispersity and reproducibility were recurring issues. SAXS measurements were performed on MCs at different pH. Although the samples were polydisperse, local maxima were observed in the structure factor plots. With more monodisperse samples, we will be able to gain structure factor information, which will hopefully prove that pH-induced polymer swelling of the MCs induces a separation of neighboring AuNPs in the polymer matrix.

To deal with the polydispersity issue encountered in chapter three, in chapter four, microfluidics was proposed as an alternative to generating Pickering emulsions. We saw how small length scales led to better control and productivity of the process. We reviewed the literature to identify the parameters to consider when setting up the experiments. These included the residence time of the droplets and the concentration of the particles used to stabilize the emulsions. Our objective was the fabrication of a PDMS chip capable of producing stable oil-in-water Pickering emulsions. We saw why PDMS was an interesting choice to build to chip. PDMS was treated to render it hydrophobic and different flow rates were tested to find flows conducive to a dripping regime. In our case, dripping occurred when the ratio between continuous phase flow rate Q w and dispersed phase flow rate Q o was Qw Qo > 1. AuNP adsorption appeared to be limited to the rear of the droplets. To maximize the number of NPs per surface area and thus the stability of Pickering emulsions, flow diagrams were constructed to find flow rate pairs yielding small and spherical droplets. Using diluted AuNP solutions as the continuous phase resulted in more coalescence, highlighting the importance of gold in droplet stability.

The last chapter introduced photopolymerization, which could polymerize the acrylate monomers in the emulsion droplets much faster than thermally initiated polymerization, making it practical to use with microfluidics. Examples from literature showed the usefulness of UV curing. We were reminded of the potential problems due to AuNPs, since they absorb and scatter incident light. The performance of the photoinitiator HMP was 148 tested first off-chip, then it was used in the microchip environment. We saw that depending on the location of UV curing and the phase containing the photoinitiator, clogging occurred in the chip. Scattering due to gold may have induced polymerization and propagation even in areas not directly exposed to UV. Proper imaging of MCs in optical microscopy or SEM required removal of excess AuNPs, via centrifugation and washing. The images suggested possible damage of the material either due to centrifugation or vacuum. As suspected, gold probably hindered polymerization, since UV curing on SDS-stabilized droplets yielded many polymer spheres within seconds. Yet gold was needed to stabilize the Pickering emulsion. A two-step approach was adopted: first, the Pickering emulsion was created. Then, excess AuNPs were washed from the droplets, and UV curing is performed, off-chip. This also prevented unwanted polymerization in the chip. MCs were obtained but they were too fragile and not colored enough. This was likely due to a lack in AuNP adsorption. Nevertheless UV curing was successful, and future tuning of the conditions will help us produce more stable MCs.

A number of experiments could help better understand the optomechanics of the MCs and optimize their production. Conducting more SAXS measurements with more monodisperse samples will reveal information about the relative positions between AuNPs at the interface, and their potential displacement during swelling and shrinking of the polymer shell. Real time color measurements of droplets will link colors to degree of AUNP adsorption. Color measurements of MCs can likewise give a color scale by which local pH or local strain can be estimated. We hypothesized that bluer MCs were due to denser adsorption of AuNPs on their surface. However blue MCs were primarily observed at pH 1. If we want to use the MCs with other products, e.g. fill them with a dye or embed them in a polymer, the highly acidic conditions should be taken into consideration. We will have to find ways to promote AuNP adsorption at the droplet interface at higher pH. Adding salts may reduce electrostatic repulsion between AuNPs. Further characterization of the polymer shell of the MCs is also necessary to better comprehend its physico-chemical properties. We do not know the degree of polymerization or the lengths of the polymer chains.

To improve the microfluidic production of MCs, a few modifications will help. New chip designs could promote more adsorption of AuNPs at the droplet interface. For instance, if the channel lengths are increased, the residence time can be lengthened as well. Or, we can reduce the nozzle dimensions at the flow-focusing junction to generate smaller droplets. Different quantities of photoinitiator, monomer, and Neobee and different intensities of UV should guide us to making better MCs via photopolymerization. The adsorption kinetics could be studied in detail. Perhaps using a fluorescent marker could trace the movement of AuNPs on the droplets as they move in the PDMS chip. Also, establishing a dimensionless number based on the flow diagrams will provide an easy way to find a dripping regime where droplets are slow-moving and spherical to maximize AuNP adsorption.

Once the ideal settings for Au-acrylate MCs in microfluidics are found, the microchip can be printed multiple times to enable parallel production of droplets, essentially scaling up. In addition, size-sorting of droplets could be an added feature: this will give us not only more monodispersity, but more MC sizes to choose from, should we want to use them elsewhere.

Meanwhile, the ultrasonication method could still be pursued as well because as we have seen, apart from the polydispersity of the objects, the MCs function as intended, i.e. they respond quickly to local pH changes. A filter or membrane could be implemented to separate different-sized MCs. If AuNP synthesis can be scaled up, then scale-up of the MC production via ultrasonication will be very possible.

After using either method of emulsification, with a large quantity of monodisperse MCs, we can introduce them in polymer films such as hydrogels, to then spread on surfaces. Each MC will act as an individual sensor, and local changes in strain or pH will be detected in a very sensitive manner. As mentioned earlier, many applications can be found for such a hydrogel. We could easily detect biofilm formation on surfaces and prevent infection in hospital settings. If used in food containers, the MCs could alert us of spoiled food. Now that we have a relatively safer MC (Neobee instead of toluene) we can test their effectiveness with respect to bacteria, in vivo. Elle est sensible, réutilisable, facile à fabriquer. Cependant il y a des améliorations à faire, notamment si on veut aller vers une chimie "verte" pour fabriquer ces MC.

A.2 Synthèse de nanoparticules d'or

Comme les MC sont à base d'émulsion Pickering d'or, il faut caractériser les NP qui seront utilisées. La résonance plasmonique dépend de la taille et de la forme des NP donc il est important de connaître ces aspects. Peut-on contrôler les différentes propriétés des NP ?

Au COMPASS nous avons fait des AuNP avec l'acide ascorbique comme réducteur. Cette méthode présente plusieurs avantages par rapport aux protocoles conventionnels. L'interaction entre l'acide ascorbique et l'or est moins forte que celle entre le CTAB et l'or, par exemple ; on peut donc faire des échanges de ligand plus facilement. Il y a aussi moins d'étapes lors de la synthèse. Pour la méthode traditionnelle à la Turkevich il faut créer des "graines" d'or qu'on fait ensuite croître. La méthode de COMPASS est plus simple car on ne fait qu'une seule réduction avec de l'acide ascorbique.

Les NP ont été synthétisées puis leur distribution de taille a été évaluée. En utilisant la technique SAXS (diffusion des rayons X aux petits angles, small angle X-ray scattering), on a estimé la distribution de taille aussi. Quand on compare les tailles mesurées à partir d'images de microscopie électronique en transmission (MET) et les données SAXS, on a des résultats similaires à 5-10 nm près.

La taille des NP était indépendante de la quantité d'agent de coiffage PVP-DADMAN. Quand la quantité d'acide ascorbique augmentait, la taille des NP semblait augmenter aussi, probablement dû à un changement en pH et donc un changement de réactivité entre le sel d'or et l'acide ascorbique lors de la synthèse des NP. Le temps de chauffage et d'agitation n'affectait pas le résultat. Les NP ont été synthétisées 100 mL à la fois. Augmenter le volume à 500 mL n'a donné le même type de NP ni en couleur ni en taille.

Ces résultats peuvent nous guider à mieux contrôler et prévoir la taille des NP. Passons aux MC qui en utilisent.

A.3 Microcapsules Au-acrylate

Comme on l'a dit au début, nous cherchons une voie moins polluante pour faire des MC. Nous souhaitons également essayer de différentes conditions pour obtenir des MC monodisperses.

Les MC conçues par COMPASS avaient comme phase huile le toluène, qui est connu pour sa toxicité. Si nous voulons utiliser les MC dans un cadre sûr, il faudra trouver un alternatif au toluène. Nous avons choisi Neobee, le nom commercial d'un mélange de triglycérides végétaux. Neobee est déjà utilisé dans les cosmétiques et l'alimentation. En comparant les propriétés physiques telles que la densité et la viscosité entre le toluène et Neobee, ce dernier semblait être un bon substitut du premier.

Les MC à base de Neobee étaient plus petites que celles faites avec du toluène ; le changement de couleur en fonction de pH a été confirmé avec Neobee comme pour le toluène.

L'amorceur que nous utilisions, l'azobisisobutyronitrile (AIBN) est soluble dans l'huile. Nous voulions tester un amorceur soluble dans l'eau, le persulfate de potassium (KPS), pour comparer les performances mais aussi car si les MC seront remplies d'un colorant ou tout autre agent, nous voulions éviter des réactions entre la charge des MC et l'amorceur. Il s'est avéré que le KPS a marché aussi bien que l'AIBN pour faire des MC. Par contre, la taille des MC ainsi obtenues étaient plus grandes. Sans amorceur nous avons eu des MC aussi.

Préparer les MC à pH différents ne changeait pas la taille, mais la couleur : les MC faites à pH 1 étaient bleues alors que celles faites à pH 3 et pH 4 étaient plus ou moins rouges ou mauves. Il y a deux facteurs qui contribuent à la couleur des MC : le taux d'adsorption des NP et le gonflement/rétrécissement de la croûte polymérique des MC. Or, à pH 4 nous étions toujours en-dessous du pK a des acides acryliques donc le gonflement n'était pas en jeu. A bas pH, la répulsion entre AuNP diminuait, et leur adsorption était plus facile à l'interface huile/eau.

Des mesures en SAXS ont été réalisées avec des MC à différentes pH. Puisque nous pensons que la distance entre NP voisines est responsable pour la couleur des MC, SAXS peut-il révéler quelque chose sur l'interaction des NP ? Les résultats suggèrent des facteur de structure qui changent en fonction du pH. Ceci dit les MC étaient trop polydisperses pour donner des signaux bien définis.

En effet la polydispersité et la repdroducibilité étaient problématiques quand nous préparions les émulsions de Pickering par ultrason. Nous avons donc proposé un moyen donnant plus de contrôle sur le processus : la microfluidique.

A.4 Emulsions de Pickering en microfluidique

Faire des émulsions en microfluidique est une méthode complémentaire aux ultrasons et apporte plusieurs avantages. On peut réaliser des expériences avec moins de volumes. Il est facile de contrôler l'aspect des gouttelettes d'émulsion. Il est possible d'étudier in situ et en temps réel la cinétique d'adsorption des nanopartcules d'or sur les gouttelettes.

Cependant, avant de faire des émulsions en microfluidique, il faut s'assurer des conditions propices à notre système : une émulsion de Pickering huile-dans-eau. Comment maximiser l'adsorption des AuNP, et comment empêcher la coalescence des gouttelettes ?

Des puces ont été conçues en polydiméthylsiloxane (PDMS) car il est facile à travailler. Mais il n'est pas idéal pour une émulsion huile-danseau, puisqu'il est hydrophobe. Si nous voulions utiliser le PDMS, il fallait le rendre hydrophile. Nous avons testé donc trois traitements de surface différents : l'eau pure, la polyvinylpyrrolidone (PVP) et la PVP-DADMAN qui est un polymère dérivé de la PVP, fait par Solvay et utilisé comme agent de coiffage des NP. En faisant des mesures de colorimétrie, la PVP a donné le meilleur résultat des trois traitements, i.e. elle laissait le moins d'AuNP s'adsorber sur les parois en PDMS.

Ensuite, un diagramme d'écoulements a été construit. Ce diagramme montrait les régimes d'écoulements observés à des débits de phase continue (solution d'AuNP) et de phase dispersée (Neobee) différents. Il y a trois régimes : le dripping, le jetting et le co-flow. Quand les gouttelettes de phase dispersée se détachent à la source d'injection dans la puce, on parle de dripping. Le jetting est quand la phase dispersée sort de la source d'injection comme un jet ou fil qui se déstabilise et forme des gouttelettes. Le co-flow désigne un régime ou les deux phases s'écoulent de manière concentrique. Le dripping était privilégié ici. Si on appelle Q w le débit de la phase continue et Q o celui de la phase dispersée, si le rapport Qw Qo était supérieur à 1, on était dans le régime de dripping.

En étudiant les gouttelettes générées, on remarquait que l'arrière des gouttelettes était principalement colorée. Cela suggère que les AuNP s'y adsorbaient principalement. Ce phénomène pourrait être expliqué par une recirculation des NP de l'avant vers l'arrière des gouttelettes quand ces dernières avancent dans la puce. Nous avions alors une contrainte géométrique : la partie colorée des gouttelettes restait le même mais les gouttelettes pouvaient être plus ou moins longues. Les NP recouvraient les gouttelettes une fois qu'elles entraient dans des canaux plus larges, donc il y avait plus de NP par surface pour les gouttelettes courtes.

Nous souhaitions alors des gouttelettes plus sphériques et plus petites car cela allait augmenter le rapport surface sur volume. A la manière des diagrammes d'écoulement, des diagrammes listant les tailles des gouttes, leur rapport de forme, et d'autres paramètres ont été faits pour des débits de phase continue et dispersée donnés. Il fallait faire des compromis quand on choisissait ces débits. Par exemple, un temps de résidence plus long donnait plus de temps aux NP à s'adsorber sur les gouttelettes, mais les gouttelettes étaient alors plus grandes. Diluer la solution d'AuNP entraînait plus de coalescence.

Nous voyons qu'il y a beaucoup de paramètres à considérer si nous voulons améliorer la stabilité des gouttelettes. Néanmoins le contrôle des tailles était excellent, surtout comparé aux gouttelettes générées par ultrason.

A.5 Photopolymérisation en microfluidique

Des émulsions de Pickering ont été fabriquées en microfluidique mais le vrai but était de faire des MC Au-acrylate avec cette technique. Or la polymérisation réalisée avant nécessitait un chauffage à 65 °C pendant 2h. Ceci n'était pas pratique en microfluidique. Nous avons alors pensé à un autre type de polymérisation : la photopolymérisation.

La photopolymérisation peut se faire soit directement en irradiant les monomères soit via un photosensibilisateur. La photopolymérisation prend moins de temps qu'une polymérisation initiée par chaleur et se réalise à une température plus basse. Ce était donc un moyen pratique de faire nos MC.

Ceci dit, si l'échantillon à photopolymériser est trouble ou contient des objects diffusants, cela devient plus difficile. Or la solution d'AuNP était justement opaque et diffusante. Nous avons testé tout de même cette nouvelle méthode de polymérisation car si nous y arrivions nous pouvions profiter pleinement de la monodispersité des MC fabriquées en microfluidique, en moins de temps qu'avec les ultrasons.

Pour voir si la photopolymérisation fonctionnait, des tests ont été réalisés dans des vials à des pourcentages massiques différents de photosensibilisateur, ici le HMP. Pour simplifier Neobee n'était pas ajouté encore dans la phase dispersée, il s'agissait donc de méthacrylate de méthyle (MMA) et d'acrylate de butyle (BA). Le HMP a fonctionné et les monomères se sont polymérisés.

La photopolymérisation étant démontrée, on a passé des vials aux puces PDMS. Des gouttelettes d'acrylate et de HMP ont été générées dans les puces. La phase continue était une solution d'AuNP. Des gouttelettes ont été lavées en sortie par centrifugation. Ceci enlevait les NP en excès autour des polymères et aidait à mieux les observer en microscopie optique ou MET. Les images montraient des gouttelettes mauves avec des surfaces plus ou moins lisses (on remarque des "bosses" ou "lobes") et une basse concentration de polymères. Les inhomogénéités étaient-elles dues à la solution d'AuNP ? La centrifugation et le vide lors de la préparation des échantillons pour la MET ont-ils détruit les polymères ?

Pour voir si l'or influence le degré de polymérisation, une solution de SDS a été utilisé comme phase continue. Comme la solution était transparente il y aurait eu moins de problèmes de diffraction ou d'absorption de lumière. Beaucoup de billes de polymère été obtenues.

Neobee a été introduit à la phase dispersée car nous voulions des capsules, pas des billes de polymère. Il est possible que les AuNPs aient plus d'affinité pour le Neobee que pour les acrylate, et s'adsorbent mieux sur la surface des gouttelettes.

Le HMP est soluble dans l'huile mais il est aussi soluble dans l'eau (certes de façon très limitée). Si nous introduisons le HMP dans la phase continue, il réagirait principalement avec les monomères à l'interface des gouttelettes, et peut-être cela donnerait de meilleurs MC. Les résultats suggéraient le contraire : on voyait un "sac" dans la puce, qui bloquait le canal et empêchait l'obtention de gouttelettes car ces dernières se sont fusionnées avec le sac. La surface des gouttelettes avait l'air peu homogène ; la polymérisation aurait eu lieu de façon irrégulière.

Les images des échantillons avec HMP dans la phase dispersée montraient que malgré le lavage il y avait toujours des AuNP autour des MC. Qui plus est, les images en MET montraient des structures aplaties : peutêtre les capsules n'étaient pas assez résistantes aux forces mécaniques. Un lavage sans centrifugation aurait été une méthode plus douce. Avec une photopolymérisation hors puce on aurait eu moins de "sac" ou bouchage des canaux.

Le remède testé était de faire les gouttelettes, les collecter dans une solution PVP-DADMAN + acide ascorbique pour les laver, puis photopolymériser. On pouvait utiliser plus de volume pour les deux phases pour générer plus de gouttelettes.

Les MC flottaient, pourtant la densité de l'or est environ 19 fois celle de Neobee. Il n'y aurait pas eu suffisament d'AuNP qui s'adsorbent à la surface des gouttelettes pour les alourdir. La couleur des MC corroborait ce manque d'adsorption aussi car on ne voyait que rarement des bleues. Elles étaient plutôt mauves ou roses. Changer le pH n'induisait pas de changement de couleur chez les MC.

En conclusion la photopolymérisation des émulsions fonctionnait mais on devra trouver les bonnes conditions pour créer des MC stables et qui changent de couleur. Le défi est d'adsorber en or mais en même temps réduire les problèmes liés à la diffraction et l'absorption.

A.6 Conclusion

Nous avons vu qu'il était possible de changer la synthèse des MC vers une chimie plus verte. Nous avons aussi vu quels paramètres influençaient la distribution de taille des AuNPs et des MCs et la couleur des MC.

La microfluidique semble prometteuse comme méthode d'émulsification car elle permet de mieux contrôler le processus et la taille des MC. Toutefois, il y a du progrès à faire : il faudra charger les gouttelettes avec plus d'AuNP afin de donner de la couleur et de la stabilité aux MC Au-acrylate. La cinétique d'adsorption des AuNP dans la puce reste à être étudiée.

Des mesures en SAXS et des observations en temps réel de l'adsorption révéleront plus d'informations sur la cinétique de l'adsorption et les interactions entre NP. Ni le degré de polymérisation des acrylates des MC ni la longueur des chaînes de polymères ne sont connus encore.

Si on pourra générer des MC à grande échelle, on pourra les mettre dans des hydrogels et créer des films qui changent de couleur localement en 156 fonction de stimuli mécanique ou chimique. Ce genre de dispositif pourra être utilisé pour détecter des contaminations bactériennes dans un milieu médical ou dans l'alimentation pour savoir si un produit ne donnera pas d'intoxication.

4

  Generation of Pickering emulsions using a microfluidic system 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Fluid mechanics at the microscale . . . . . . . . . . . 4.1.2 (Pickering) emulsions in microfluidics . . . . . . . . . 4.2 Microfabrication of channels . . . . . . . . . . . . . . . . . . . 4.2.1 Photolithography . . . . . . . . . . . . . . . . . . . . . 4.2.2 Printing PDMS chips . . . . . . . . . . . . . . . . . . 4.3 Surface treatment of microchannels . . . . . . . . . . . . . . . 4.3.1 PDMS: a practical polymer . . . . . . . . . . . . . . . 4.3.2 On the utility of surface treatment . . . . . . . . . . . 4.3.3 Experiments . . . . . . . . . . . . . . . . . . . . . . . 4.4 Formation of Pickering emulsion in microchannels . . . . . . . 4.4.1 Flow diagrams . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Adsorption of nanoparticles . . . . . . . . . . . . . . . 4.4.3 Mapping key parameters . . . . . . . . . . . . . . . . . 4.5 Coalescence observations . . . . . . . . . . . . . . . . . . . . . 4.5.1 Imaging in and out the chip . . . . . . . . . . . . . . . 4.5.2 Influence of NP concentration . . . . . . . . . . . . . . 4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Photopolymerization of Au-acrylate microcapsules in a microfluidic chip 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 A need for a new method of polymerization . . . . . . 5.2 Photopolymerization in microfluidics . . . . . . . . . . . . . . 5.3 Acrylate-in-water emulsions . . . . . . . . . . . . . . . . . . . 5.3.1 Preliminary tests . . . . . . . . . . . . . . . . . . . . . 5.3.2 On-chip UV curing . . . . . . . . . . . . . . . . . . . .

Figure 1 -

 1 Figure 1 -Ramesseum, Thebes. Joseph-Philibert Girault de Prangey, 1844.1

Figure 1 . 1 -Figure 1 . 2 -

 1112 Figure 1.1 -Polyurethane film containing cyano-OPVs before (a) and after (b) stretching. Images taken under excitation by UV light of 365 nm wavelength. [5]

Figure 1 . 3 -

 13 Figure 1.3 -Schematic drawing of microcapsules releasing DCF. [8]

Figure 1 . 5 -

 15 Figure 1.5 -(a) Colorless spiropyran becomes colored merocyanine when its spiro C-O bond is opened. (b) Tensile loading results show color intensity increases with plastic strain. [10]

Figure 1 . 6 -

 16 Figure 1.6 -(a) Mechanism of gated photoswitching. Mechanical force is a prerequisite to obtain colored diarylethene (DAE). (b) UV-vis spectra of polymer. Color change is observed after ultrasonication and exposure to UV light. [10]

Figure 1 . 7 -

 17 Figure 1.7 -(a) Schematic representation of process to obtain an SU-8 inverse opal. (b) Reflectance spectra of the inverse opal prepared from NPs of different sizes. (c-e) Optical microscopy images of opals constructed from Si NPs of diameters (c) 320, (d) 285, and (e) 238 nm. Inset: cross-sectional FE SEM images of each opal.[14]

Figure 1 . 9 -Figure 1 . 10 -

 19110 Figure1.9 -(a) Pulling on a PDMS film covered in a monolayer of AuNPs causes the distance between NPs to increase in the stretching direction and decrease in the direction perpendical to the stretching. Color of the film goes from purple-red (b)to blue-violet(c). Images were taken with light polarized perpendicular to the applied strain.[17]

Figure 1 .

 1 Figure 1.13 -UV-vis spectra of (a) gold and (b) silver nanoparticle solutions. [27]

Figure 1 .

 1 Figure 1.14 -UV-vis spectra of different sizes of AuNPs, normalized at (a)peak extinction[28] or at (b) 400 nm.[29]

Figure 1 .

 1 Figure 1.15 -(a) TEM of AuNPs of different shapes. A) Nanospheres, B) nanocubes, C) nanobranches, D) nanorods (aspect ratio=2.4±0.3), E) nanorods (aspect ratio=3.4±0.5. (b) UV-vis spectra of the NPs.[30]

Figure 1 . 17 -

 117 Figure 1.17 -Extinction spectra of 40 nm gold nanosphere pairs for different values of s: ∞ (a), 7 (b), 4 (c), 2 (d), 0.7 (e), 0.4 (f), 0.2 nm (g). Light used is unpolarized. Number of harmonics used in calculation: 30. [32]

Figure 1 . 18 -

 118 Figure 1.18 -Contact angle θ can affect the nature of the Pickering emulsion. [27]

Figure 1 . 19 -Figure 1 . 20 -

 119120 Figure 1.19 -Effect of pH on nature of emulsions stabilized by partially hydrophobic silica particles.[START_REF] Binks | Effects of oil type and aqueous phase composition on oil-water mixtures containing particles of intermediate hydrophobicity[END_REF]

Figure 1 .

 1 Figure 1.21 -(a) Water trapped on particle surface causes repulsion in oil phase. (b) Solvated charges and counterions create dipoles, which cause particles to repel each other.[START_REF] Mcgorty | Colloidal self-assembly at an interface[END_REF]

Figure 1 .

 1 Figure 1.22 -(A)SEM image of dried, 10 µm diameter colloidosome composed of 0.9 µm diameter PS spheres. [62]

Figure 1 .

 1 Figure 1.24 -(a) Emulsion prepared with 0.02 M of gold and pH 10. (b) Microcapsules after polymerization, 0.02M gold and pH 10.(c) Microcapsules and (d) enlarged surface of microcapsules (0.02 M gold, pH 10) obtained with SEM. (a-c) scale bars 10 µm, (d) scale bar 100 nm.[27]

Figure 1 .

 1 Figure 1.25 -(a) AuNPs stabilized at pH 1 in a toluene-in-water emulsion. (b) Au-Si microcapsules. (c,d) SEM images of Au-Si microcapsules at pH 1. Inset of (c) shows NPs on surface. Inset of (d) shows the silica shell. (a-b) scale bars 20 µm. (c) scale bar 10 µm, inset scale bar 100 nm. (d) scale bar 10 µm, inset scale 1 µm.[27]

Figure 1 .

 1 Figure 1.26 -(a) PVA film doped with gold-silica microcapsules. Scale bar 1 cm. (b) A single microcapsule, 20 µm in diameter.[27]

  Stretching tests A PVA film sample doped with Au-Si microcapsules was heated above glass transition temperature and stretched. The capsules became more elongated and the color change more pronounced with increasing deformation of the film (fig. 1.27).

Figure 1 .

 1 Figure 1.27 -Au-Si microcapsules embedded in PVA film before (a), during (b), and after (c) uniaxial strain. d) Schematic representation of the deformation of the microcapsules and their color change. Scale bars 20 µm.[27]

Figure 1 .

 1 Figure 1.29 -(a) Hole punched in PVA film. Optical microscopy images show capsules located far from hole (b), closer (c) and very close to hole (d).(e) Gradient of color and elongation near hole. Scale bar of (a) 5 mm, other scale bars 5 µm.[27]

Figure 1 . 30 -

 130 Figure 1.30 -Shell thickness depends on the concentration of PEOS. If it is too low, there is a risk of buckling or collapse of the capsules. Top scale bar 5 µm, other scale bars 2 µm.[27]

Figure 1 .

 1 Figure 1.31 -(a-c) Gold-methyl methacrylate-co-butyl acrylate microcapsules. (a) AuNPs stabilized at interface of toluene-in-water emulsion. (b) Microcapsules, blue due to the additional AuNPs. (c) SEM image of the microcapsules. Inset depicts AuNPs present in the polymeric shell. (d-e) Gold-methacrylate-co-butyl acrylate microcapsules. (d) AuNPs stabilized at interface of toluene-in-water emulsion. (e) Microcapsules obtained. (f) SEM image of the microcapsules. Inset shows AuNPs embedded in the polymeric shell. Scale bars 20 µm, inset scale bars 100 nm.[27]

Figure 1 .

 1 Figure 1.32 -Gold-acrylate microcapsules before deformation (a), after uniaxial (b) and biaxial (c,d) deformation.[27]

Figure 1 . 33 -Figure 1 . 34 -

 133134 Figure 1.33 -Color change of gold-acrylate microcapsules is reversible after three cycles. At pH=1, the microcapsules are blue, but after addition of NaOH solution, they become pink at pH=14. Adding HCl reverts the color to blue. Scale bars 20 µm.[27]

  2.1).

Figure 2 . 1 -

 21 Figure 2.1 -TEM image of AuNPs obtained by a two-step seeding of colloidal Au with NH 2 OH. [4]

Figure 2 . 2 -

 22 Figure 2.2 -TEM images of AuNPs obtained after successive growth steps Average diameters are (a) 66, (b) 100, (c) 139, (d) 157, and (e) 181 nm. [5]

3 99 1 Figure 2 . 3 -

 3123 Figure 2.3 -Structure of PVP-DADMAN. The polymer contains 1 unit of diallyldimethyl-ammonium nitrate (DADMAN) per 99 units of polyvinylpyrrolidone (PVP). M w =30000 g/mol, pK a =5.

Figure 2 . 4 -

 24 Figure 2.4 -Experimental setup of AuNP synthesis.

  (a) UV-vis spectra (b) Calibration curve

Figure 2 . 5 -

 25 Figure 2.5 -AuNP solution calibration. A sample was dried and weighed to estimate initial [Au 0 ], then was diluted several times to create a calibration curve.

Figure 2 . 6 -

 26 Figure 2.6 -TEM image of AuNPs obtained with the synthesis described in 2.2.2. Scale bar 100 nm.

Figure 2 . 7 -

 27 Figure 2.7 -Signals of AuNP solution diluted several times. Inset: same signals normalized with respect to concentration.

Figure 2 . 8 -

 28 Figure 2.8 -Means and standard deviations found by fitting SAXS signals of AuNP solutions. 55

Figure 2 . 9 -

 29 Figure 2.9 -Comparing NP size data obtrained from SAXS and TEM.

  (a) UV-vis spectra (normalized at 400 nm) (b) Plotting resonance peaks

Figure 2 . 10 -

 210 Figure 2.10 -Comparing different [PVP-DADMAN]. All other synthesis conditions identical to those described in 2.2.2.

  (a) UV-vis spectra (normalized at 400 nm) (b) Plotting resonance peaks

Figure 2 . 11 -

 211 Figure 2.11 -Comparing different [ascorbic acid]. All other synthesis conditions identical to those described in 2.2.2.

Figure 2 .

 2 Figure2.12 -NaOH is added to modify pH of the ascorbic acid (AA) used during synthesis.[1]

  2.15).

Figure 2 . 13 -

 213 Figure 2.13 -Comparing different synthesis times. All other synthesis conditions identical to those described in 2.2.2. Sample taken at 30 minutes gave an outlier; it is not presented here.

Figure 2 .

 2 Figure 2.14 -UV-vis spectra of a 475 mL batch and a 100 mL batch. Normalized at 400 nm, diluted 10x.

Figure 2 . 15 -

 215 Figure 2.15 -Visible difference in color between the 475 mL preparation on the left and a "regular" 100 mL preparation on the right. Jar in the background contains excess sample from 475 mL (left jar was too small).

Figure 3 . 1 -

 31 Figure 3.1 -When pH > pK a , water can enter the polymer matrix. [7]

Figure 3 . 2 -

 32 Figure 3.2 -Emulsion droplets observed at different stages during synthesis: right after 15 minutes of an ultrasonic bath (a), after an hour of stirring and heating (b). Scale bar 25 µm.

3. 3 )Figure 3 . 3 -

 333 Figure 3.3 -MC color change at (a) t=0 s and (b) t=30 s as it is washed over by a NaOH solution. Scale bar 10 µm.

  (a) Histogram bin width 2. Image scale bar 25 µm. (b) Box charts of data.

Figure 3 . 4 -

 34 Figure 3.4 -Comparing oil phases after 2h of polymerization.

  (a) Histogram bin width 2. Image scale bar 25 µm. (b) Box charts of data.

Figure 3 . 5 -

 35 Figure 3.5 -Comparing initiators in toluene after 2h of polymerization.

Figure 3 . 7 -

 37 Figure 3.7 -Depending on the pH, the AuNPs are either dispersed (red solution) or flocculated (blue solution). Inset: plasmonic peak wavelengths (circles) and ζ potential of AuNPs at [Au 0 ]= 0.57 M with respect to pH. [7]

  (a) Size differences according to pH. Histogram bin width 2. (b) Blue color is primarily observed at very low pH. Scale bar length 25 µm.

  (c) Box charts of data.

Figure 3 . 8 -

 38 Figure 3.8 -Exploring different pH and their effect on the capsules. Initiator KPS, oil phase Neobee.

  (b) Blue color is primarily observed at very low pH. Scale bar length 25 µm.

Figure 3 . 9 -

 39 Figure 3.9 -Effect of pH on MCs. Initiator AIBN, oil phase Neobee.

  (a) Box charts of data.

Figure 3 . 10 -

 310 Figure 3.10 -Effect of pH on MCs. Initiator AIBN, oil phase Neobee.

  3.11).MCs were synthesized and divided into three parts: one was unaltered, one had NaOH added to it to bring its pH to around 13, and the final sample had first NaOH then HCl added to it to bring the pH back to the original value (around 2). The intensities were divided by the baseline, i.e. the signal of the NP solution only (fig.3.11).

Figure 3 .

 3 Figure 3.11 -SAXS measurements of MCs exposed to acids and bases. I N P is the intensity of the AuNP solution alone, and I 0 is the intensity of pure water.

  4.2)[11][12].

Figure 4 . 1 -

 41 Figure 4.1 -Flow-focusing geometry. Two continuous phase flows pinch the central dispersed phase and forces drop detachment. [9]

Figure 4 . 2 -

 42 Figure 4.2 -(a-c) Real-time images of the microfluidic emulsification process of the double emulsions with single, double, and triple cores, respectively. Multiple capillaries are placed in an array on the left, and each one can release its contents to form w/o/w emulsions. Scale bar 500 µm.(d-f) Microscopy photographs of smaller double emulsions at high flow rates with single, double, and triple cores, respectively. Scale bar 300 µm.[11]

  (a) Schematic of particle stabilized drop formation. Chip material is glass. (b) Variation in coalescence time (t c ) with particle concentration (C p ) for oil droplets brought into contact shortly after formation, as shown in the inset. Q o = 0.05 mL/h and Q o /Q aq = 1/3.

Figure 4 . 3 -

 43 Figure 4.3 -Concentration of silica particles in the aqueous phase affects the time it takes for coalescence to occur between two dodecane droplets.[14]

  (a) Layout of microfluidic circuit with q c and q d denoting flow rates of continuous and dispersed phases at respective inlets, and q t denoting total flow rate at outlet of chip. Regions where images are recorded are indicated by rectangles: (A) Tjunction, (B) inlet collision channel and (C) outlet collision channel. (b) Light microscopy images of sunflower o/w emulsions (5% w/w CLP of which flow rate indicated at the left) at the end of the coalescence channel, and polarized images after collection from microfluidic set-up (right).

Figure 4 . 4 -

 44 Figure 4.4 -Coalescence depends on continuous phase flow rates. [14] 90

Figure 4 . 5 -

 45 Figure 4.5 -Number of coalescence events as a function of the continuous phase flow rate (µL/min) for sunflower O/W emulsions formed with various concentrations of CLPs in the continuous phase. The dispersed phase flow rate was 1 µL/min.No coalescence events are observed for phosphate buffer and the 5% w/w CLP dispersion.[15]

  et al. (cm vs. mm scale) so the AuNPs should have enough time to diffuse and adsorb onto the oil/water interface as the droplets advance in the chips.

Figure 4 . 6 -

 46 Figure 4.6 -Droplet stability as a function of incubation length L: size distribution of droplets in the coalescence chamber. Surfactant concentration C = 10 -3 g/g is constant. (a) L = 1 mm (τ = 7 ms); the emulsion is unstable (droplet area before coalescence A 0 = 1914 µm 2 ). (b) L = 5 mm (τ = 36 ms); the proportion of fused droplets is less than 10 -3 (A 0 = 2028 µm 2). Increasing the length of the channel after production helps stabilize emulsions.[16]

Figure 4 . 7 -

 47 Figure 4.7 -Basic process of photolithography. [1]

Figure 4 . 8 -

 48 Figure 4.8 -Plasma cleaner in use, with the glass slide and PDMS exposed to the plasma. Both exposed sides will be brought together for plasma bonding.

Figure 4 . 9 -

 49 Figure 4.9 -Polydimethylsiloxane (PDMS).

  4.10. Since AuNPs are needed to stabilize the Pickering emulsions, unwanted adsorption of AuNPs on the PDMS walls (fig. 4.11) can be prevented with surface treatment as well.

Figure 4 . 10 -

 410 Figure 4.10 -Two possible bondings between PDMS and PVP: covalent bonds (top path) or hydrogen bonds (bottom path).[22]

Figure 4 .

 4 Figure 4.11 -A used PDMS chip showing stains caused by AuNP solution injected during experiments (cf fig. 4.14). Scale bar 1 cm.

Figure 4 . 12 -

 412 Figure 4.12 -Measuring green light transmittance for different types of surface treatment.

4. 4 . 1

 41 Flow diagramsThree flow regimes Reproducibility and predictability of behavior are part of the appeal of using microfluidics. But what flow rate settings are conducive to stable, monodisperse emulsions? Depending on the flow rates, three flow regimes can be observed: dripping, jetting and co-flow (fig. 4.13) [27].

Figure 4 . 13 -

 413 Figure 4.13 -Three different regimes have been observed in various dropletforming geometries: (a) coaxial, (b) flow-focusing, (c) T-junction. [27]

Figure 4 . 14 -

 414 Figure 4.14 -Schematic of the chip used to construct flow diagram. Flow direction is from left to right.

Figure 4 . 16 -

 416 Figure 4.16 -Arrow denotes direction of flow. Note the shaded rear of the droplet (circled). Scale bar 100 µm.

Figure 4 . 17 -

 417 Figure 4.17 -Montage of droplets entering coalescence zone (cf fig. 4.24), 5 ms interval between images (top-bottom, left-right). Scale bars 100 µm.

Figure 4 . 18 -

 418 Figure 4.18 -Particles adsorb and pack at the rear of the bubble, leadingto a no-slip (NS) boundary condition at the rear bubble interface compared with a shear-free condition (S) at the leading edge. Added drag from the immobile interface reduces the bubble speed.[30]

Figure 4 . 19 -

 419 Figure 4.19 -Sketch of the droplet in the microchannel as described by Baret et al. In the frame of the droplet, the rotating motion of the oil results in an almost zero net flux of the surfactant toward the droplet interface.In the vicinity of the droplet, diffusion enables micelles to reach the droplet interface. The number of surfactant molecules in the volume, v, surrounding the droplet is sufficient to stabilize the interface by simple diffusion of the micelles over the distance ε.[16]

  NP solution flow rate Q w + Neobee flow rate Q o = total flow rate Q -Frequency of droplet formation f d -Droplet volume V d =Q o /f d -Residence time t r and droplet speed v d -Droplet aspect ratio A flow diagram was constructed, on which the values of the parameters were displayed. Only dripping regime was investigated because the other flow regimes do not generate consistent droplets.

Figure 4 . 20 -

 420 Figure 4.20 -Droplet generation frequency. Bigger circles mean higher frequencies.

Figure 4 . 21 -

 421 Figure 4.21 -Droplet volumes.

Figure 4 . 22 -

 422 Figure 4.22 -Residence time of droplets.

Figure 4 . 23 -

 423 Figure 4.23 -Aspect ratios of droplets.

1 .

 1 A hydrophilic PDMS surface for o/w emulsions and less AuNPs on the chip walls 2. Dripping regime 3. Droplets with smaller surface area 4. More spherical droplets 5. Longer residence times Now let us see if tuning these parameters contributes to stable emulsions. Instead of the flow-focusing part of the chip, we will check what happens downstream and outside the chip.

4. 5

 5 Coalescence observations 4.5.1 Imaging in and out the chip After reviewing the flow diagrams and "maps" of the parameters discussed in section 4.4.3, the flow rate pair (Q w , Q o )=(8,2) (in µL/min) seemed the most suitable. Meanwhile, a new chip was designed. A wide and long coalescence zone was added to slow the droplets down and thus increase their time of residence, hopefully aiding in droplet stability. The design is shown in fig. 4.24.

Figure 4 . 24 -

 424 Figure 4.24 -Newer chip design with wider and longer coalescence zone.

  (a) Droplet sizes for emulsion obtained by chip. (b) Droplet sizes for emulsion obtained just after sonication.

Figure 4 . 26 -

 426 Figure 4.26 -Comparing size distributions between methods of emulsification. PDI of sonicated droplets: 0.45. PDI of chip-generated droplets: 0.19.

  4.27b). It is essential to use concentrated AuNP solution.

  (a) Fewer coalescence events are observed with higher concentrations of AuNP solution. Red square denotes where images were taken on chip. Canal width 1000µm. Q w =8 µL/min, Q o =2 µL/min.

  (b) Size distributions and Gaussian fits of the data shown above. PDI found for c 0 /10, c 0 /4, c 0 /2, and c 0 are respectively: .42, .21, .26, .19.

Figure 4 . 27 -

 427 Figure 4.27 -Dilution of AuNP solution causes more coalescence and polydispersity. 116

Figure 5 . 2 -Figure 5 . 3 -Figure 5 . 4 -

 525354 Figure 5.2 -Schematic of a polymer-analogous gelation to form microgels with droplet microfluidics. (A) Sketch of a microfluidic device emulsifying a pNIPAAm solution in water. Subsequent crosslinking of the chains by dimerizing reactive side groups with a photochemical reaction forms polymer microgels. (B) UV-induced crosslinking of dimethylmaleimide (DMMI) functionalized polymers.In aqueous media, the reaction is mediated by a triplet sensitizer, thioxanthone-2,7-disulfonate. Two isomeric types of DMMI-dimers are formed, each constituting a covalent junction between the precursor chains.[16]

Figure 5 . 5 -Figure 5 . 6 -

 5556 Figure 5.5 -Schematic diagram of the flow-focusing microfluidic device and the typical droplet formation process in channel. (a) PEGDA droplets are produced in the nozzle zone, the dispersed phase (PEGDA solution) is injected from inlet A and the continuous phase (hexadecane) is imported from inlet B. Left inset: enlargement of nozzle zone. Black dots in disperse phase represent QDs. The other part is the UV exposure zone with gradually widening serpentine for in situ polymerization of PEGDA droplets. Right inset shows solidifying process of PEGDA beads in channel. (b) Optical micrographs of the generation process of a single droplet (Q c = 5 µL/min and Q d = 30 µL/min). Scale bar 100 µm.[19]

Figure 5 . 8 -

 58 Figure 5.8 -Structure of HMP.

Figure 5 . 9 -

 59 Figure 5.9 -Schematic of chip used in the experiments. a) continuous phase inlet b) dispersed phase inlet c) flow-focusing junction d) serpentine e) coalescence zone f) area where UV is applied, right before g) outlet. Chip height 100 µm, serpentine width 100 µm, coalescence zone width 1000 µm.

Figure 5 .

 5 Figure 5.10 -OM image of collected sample (cf section 5.3.2), tranmission mode. Scale bar 100 µm.

Figure 5 .

 5 Figure 5.11 -SEM image of collected sample (cf section 5.3.2). Scale bar 1 µm.

  (a) Polymer balls in coalescence zone of chip. No coalescence observed. (b) Sample collected from chip and placed on glass slide. Off-chip coalescence was not observed.

Figure 5 .

 5 Figure 5.12 -Using SDS solution instead of AuNP solution gives many stable polymer balls. All scale bars 300 µm.

Figure 5 . 15 -

 515 Figure 5.15 -MCs collected from the bottle after UV curing. Image taken in transmission mode. Scale bar 100 µm.

Figure 5 . 16 -

 516 Figure 5.16 -MCs burst and leave behind a colored "shell," as seen in the background. Image taken in transmission mode. Scale bar 100 µm.

1

 1 Etat de l'artDans la vie quotidienne, nous utilisons de plus en plus de polymères. Il est important de savoir quand et où les polymères sont endommagés afin de prévenir des accidents et d'améliorer les produits futurs ; cela est possible grâce à des systèmes de détection tels qu'un capteur mécanochrome.Un capteur mécanochrome change de couleur en fonction des contraintes mécaniques qui lui sont exercées. Il en existe plusieurs exemples. Les excimères sont des dimères qui fluorescent à des longueurs d'ondes différentes à leur version monomérique. Des capsules contenant des colorants peuvent indiquer des fractures quand elles sont cassées et relâchent leur contenu dans le milieu. Les mécanophores comme le spiropyran sont des molécules dont les réactions sont déclenchées par des forces mécaniques. Les cristaux photoniques laissent certaines longueurs d'ondes passer et en bloquent d'autres ; changer leur conformation change la couleur. Les objets plasmomécaniques absorbent certaines longueurs d'onde et en diffusent d'autres. Au COMPASS (laboratoire collaboratif entre le CNRS, Solvay et UPenn) nous avons conçu un nouveau capteur. Il s'agit de microcapsules (MC) mécano chromes et plasmoniques. Ces MC peuvent changer de couleur en fonction des forces mécaniques ou du pH. Pour faire ces MC, d'abord on fait une émulsion de Pickering huile-danseau, stabilisée par des nanoparticules d'or (AuNP). La phase huile contient des monomères d'acrylate, qui sont polymérisés pour former une capsule. Les AuNP sont donc dans une croûte polymérique. La couleur des MC dépend de la distance entre NP. Si le pH autour des MC devient basique, il y a déprotonation des acides acryliques et donc entrée d'eau : les MC sont gonflées. Les NP dans la croûte s'éloignent et la couleur qu'on observe change. Ce changement de couleur est réversible puisque quand le pH redevient acide, la MC se rétrécit.Cette technologie a plusieurs avantages par rapport à d'autres capteurs.

3

  Gold-acrylate microcapsule synthesis 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Creating plasmonic polymers . . . . . . . . . . . . . . . . . . 3.2.1 Suspension polymerization . . . . . . . . . . . . . . . . 3.2.2 pH-based changes in plasmonic resonance . . . . . . . 3.3 Changing synthesis conditions . . . . . . . . . . . . . . . . . . 3.3.1 Reproducing previous experiments . . . . . . . . . . . 3.3.2 New oil phase . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Changing the initiator . . . . . . . . . . . . . . . . . . 3.3.4 Summary: effect of oil phases and initiators on capsules 3.3.5 Changing pH . . . . . . . . . . . . . . . . . . . . . . . 3.3.6 Summary: synthesis conditions . . . . . . . . . . . . . 3.4 SAXS experiments . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table

  .23)[68]. If we could create the silica shell and make AuNPs adsorb on it at the same time, more or less, we could make gold-silica microcapsules.

	Si OR + H 2 O esterification hydrolysis	Si OH + ROH
	Si OR + HO Si alcohol condensation alcoholysis Si O Si	+ ROH
	Si OR + HO Si water condensation hydrolysis Si O Si	+ H 2 O
	Figure 1.23 -Different reactions that can occur to PEOS at the interface:
	first, hydrolysis, then either alcohol or water condensation. [27]

  Shah, Jin-Woong Kim, and David A. Weitz. Monodisperse stimuli-responsive colloidosomes by self-assembly of microgels in droplets. Langmuir, 26(3):1561-1565, 2010. PMID: 19950936.[67] Chul-Suk Hong, Jae Hyung Park, Soonkoo Lee, Kun Yil Rhoo, Jong Tak Lee, and Seung R. Paik. Fabrication of protease-sensitive

and light-responsive microcapsules encompassed with single layer of gold nanoparticles by using self-assembly protein of α-synuclein. ACS Applied Materials & Interfaces, 10(31):26628-26640, 2018. PMID: 30052414.

Table 3 .

 3 .1):

	Aqueous phase	Oil phase
	AuNP solution 5 mL ([Au0]=0.2M)	Toluene 625 µL
	HCl (1M) 62.5 µL	Methyl methacrylate 0.14 g
		Butyl acrylate 0.14 g
		AIBN (0.12 M)

1 -The components of each phase of the emulsion used to create Au-acrylate MCs.

Table 3 .

 3 .2). 2 -Comparing densities and viscosities of toluene and Neobee.

	Oil	Toluene Neobee
	Density	.87	.95
	Viscosity at 25 °C (cP)	56	23

table ( table 3 .

 (3

	3).
	Initiator	Oil phase Toluene Neobee
	None	22	11
	AIBN	13	7
	KPS	23	16

Table 3 .

 3

3 -Median radii of droplets for different conditions in µm.

(a) UV-vis spectra (normalized at 400 nm) (b) Plotting resonance peaks

(a) Histogram bin width 2. Image scale bar 25 µm. (b) Box charts of data.Figure 3.6 -Comparing initiators in Neobee after 2h of polymerization.

Acknowledgments

Chapter 4

Generation of Pickering emulsions using a microfluidic system

Introduction

The method of emulsification seen so far was ultrasonication. This is an effective means to quickly obtain an emulsion but it presents several issues as well. We saw that the size and dispersity of the resulting microcapsules vary. Precise control of the process is not guaranteed. Observation of the system in real time during sonication is not feasible either. A complementary method of emulsification could be the use of microfluidic techniques. Microfluidics presents a number of advantages compared to ultrasonication. For example:

-Experiments can be run with smaller reagent volumes [1].

-Size dispersity issues are practically nonexistent and the aspect of the emulsion droplets can be fine-tuned [2]. -High-throughput systems are feasible [3].

-In situ study of emulsification is possible [2]. This could help better understand how the AuNPs adsorb at the droplet interface. Switching from ultrasonication to microfluidics as the method of emulsification is a transition from millifluidic to microfluidic scales, from turbulent flows to laminar ones, and requires engineering of the microfluidic setup to properly generate stable oil-in-water Pickering emulsions.

In this case, this means understanding the surface properties of a microfluidic chip, exploring the different flow regimes, preventing coalescence of emulsion droplets, and finding out what influences droplet stability.

What are the merits of a microfluidic approach? How does working at smaller length scales help obtain better monodispersity and control of the process? incremented or decremented and images were taken at the nozzle to evaluate the type of flow regime obtained. The results are presented in fig. Judging by the results, dripping is achieved as long as Qw Qo > 1, and transition from jetting to co-flow occurs at Qw Qo < 1 4 . The flow diagram helps us choose flow rates if we want to observe a given regime, as well as predict the regime before setting up an experiment. Flow rate ratios conducive to dripping will be prioritized i.e. Qw Qo > 1. The flow rate study helps obtain monodisperse droplets in a controlled manner. But the droplets still need be properly adsorbed by the AuNPs to form stable Pickering emulsions. The key observation here is that the majority of droplets do not coalesce. Even after collection and exposure to air, around 80% of the droplets stay intact for at least several minutes. If placed under a cover glass, they last at least a day, as long as the water surrounding them doesn't evaporate. This shows that the droplets are very stable.

Let the polydispersity index PDI be defined as P DI = σ µ where σ components are similar, scattering is reduced [22], but it is clear that gold will have very different scattering properties compared to the monomers or chip (table 5 To summarize, photopolymerization is quite practical: it requires less time, cooler temperatures, and pollutes less. However there are several caveats.

Due to the limited solubility of HMP in water, it is likely that polymerization did not evenly occur. A closer look at the surface of the droplets also suggests inhomogeneous polymerization, with crack-like features (fig. 5.13).

Imaging

Let us compare images of samples prepared with (fig. We first notice in fig. 5.14b that despite the washing, a large amount of AuNPs surround the polymer. The AuNPs are on top of the polymer, rather than embedded in it. The polymer has a flattened appearance.

From these observations, it appears that the washing was incomplete, and that the centrifugation and washing, along with vacuum and drying during SEM preparation, may have damaged the polymer. This suggests that the polymer is not resistant enough. If photopolymerization occurred so quickly that AuNPs were pushed away, unable to access the polymer matrix, they could have stayed atop the polymer as seen here.