Steven Van Kesteren

Laura Alvarez

Silvia Arrese-Igor

Angel Alegria

Lucio Isa
email: lucio.isa@mat.ethz.ch

SELF-PROPELLING COLLOIDAL FINITE STATE MACHINES

Keywords: Active matter, Colloidal molecules, Microrobots

Endowing materials with physical intelligence holds the key for a progress leap in robotic systems. In spite of the growing success for macroscopic devices, transferring these concepts to the microscale presents several challenges connected to the lack of suitable fabrication and design techniques, and of internal response schemes that connect the materials' properties to the function of an autonomous unit. Here, we realize self-propelling colloidal clusters which behave as simple finite state machines, i.e. systems built to possess a finite number of internal states connected by reversible transitions and associated to distinct functions. We produce these units via capillary assembly combining hard polystyrene colloids with two different types of thermo-responsive microgels. The clusters, actuated by spatially uniform AC electric fields, adapt their shape and dielectric properties, and consequently their propulsion, via reversible temperature-induced transitions controlled by light. The different transition temperatures for the two microgels enable three distinct dynamical states corresponding to three illumination intensity levels. The sequential reconfiguration of the microgels affects the velocity and shape of the active trajectories according to a pathway defined by tailoring the clusters' geometry during assembly. The demonstration of these simple systems indicates an exciting route to build more complex units with broader reconfiguration schemes and multiple responses towards the realization of autonomous systems with physical intelligence at the colloidal scale.

Introduction

As automation increasingly pervades our lives, new paradigms for autonomous systems across a broad range of length scales and application fields are in high demand. The recent upsurge of soft robotics is a clear example, where strong efforts are currently underway to endow robots with physical intelligence [START_REF] Sitti | Physical intelligence as a new paradigm[END_REF]. This concept aims at transferring functions such as perception, control and response from computational units to physical agents toward the realization of robotic materials, i.e. materials that can perform one or more of these functions without external logic and that can be integrated in autonomous devices [START_REF] Truby | Designing Soft Robots as Robotic Materials[END_REF]. A simple guidance to constructing these systems can be based on the concept of a finite state machine (FSM). This model describes the behavior of a system as being defined by a finite number of different internal states connected by reversible transitions triggered in response to a defined set of inputs. An FSM description perfectly befits the scope of a robotic material, in the sense of a material whose function depends on its state, e.g. solid/liquid, expanded/contracted, soft/stiff, conductive/non-conductive, etc., and which can be switched by a specific trigger, e.g. temperature, light or electromagnetic signals.

Even if the implementation of these concepts is rapidly becoming more frequent at the macroscopic scale [START_REF] Breger | Self-folding thermo-magnetically responsive soft microgrippers[END_REF][START_REF] Koohee Han | Sequence-encoded colloidal origami and microbot assemblies from patchy magnetic cubes[END_REF][START_REF] Dou | Autonomous navigation of shape-shifting microswimmers[END_REF], its extension to the (sub)-micrometric or colloidal scale still presents significant challenges. Current microscale systems focus on self-propelling particles, aka artificial microswimmers or active colloids. These systems comprise units that can spontaneously convert an energy input, either externally supplied, e.g. in the form of an electromagnetic or acoustic field, or internally accessible, such as chemical fuel, into directed motion or self-propulsion. This kind of motility is at the core of the current and future applications of microswimmers to act as delivery vehicles [START_REF] Esteban | Micromotor-enabled active drug delivery for in vivo treatment of stomach infection[END_REF][START_REF] Hortelao | Swarming behavior and in vivo monitoring of enzymatic nanomotors within the bladder[END_REF][START_REF] Birgul Akolpoglu | Magnetically steerable bacterial microrobots moving in 3d biological matrices for stimuli-responsive cargo delivery[END_REF][START_REF] Sridhar | Light-driven carbon nitride microswimmers with propulsion in biological and ionic media and responsive on-demand drug delivery[END_REF], active mixers [START_REF] Schuerle | Synthetic and living micropropellers for convection-enhanced nanoparticle transport[END_REF], or remediation agents [START_REF] Wang | Catalytic dna-functionalized self-propelled micromachines for environmental remediation[END_REF][START_REF] Wang | Highly efficient mof-based self-propelled micromotors for water purification[END_REF] , to name a few. Crucially, self-propulsion is enabled by the fact that active colloids have an intrinsic asymmetry in their geometrical or compositional properties, which defines the directionality of their motion [START_REF] Ni | Hybrid colloidal microswimmers through sequential capillary assembly[END_REF][START_REF] Dou | Autonomous navigation of shape-shifting microswimmers[END_REF]. However, this asymmetry is typically fixed during synthesis and fabrication [START_REF] Wang | Active patchy colloids with shape-tunable dynamics[END_REF], implying that synthetic microswimmers only have one internal state, and that propulsion is regulated by controlling the energy input.

The application of an FSM model to an active colloidal material requires the following features: i) a self-propulsion mechanism that depends on the particle's state, i.e. shape or internal properties; ii) the possibility to reversibly switch between different states in response to well-defined inputs; iii) the possibility to encode the states and their transitions in a deterministic fashion during fabrication.

In this work, we fulfill these requirements by fabricating artificial microswimmers constituted by colloidal clusters or "molecules" [START_REF] Soto | Self-assembly of catalytically active colloidal molecules: Tailoring activity through surface chemistry[END_REF][START_REF] Niu | Self-assembly of colloidal molecules due to self-generated flow[END_REF][START_REF] Löwen | Active colloidal molecules[END_REF], which combine responsive and non-responsive particles [START_REF] Alvarez | Reconfigurable artificial microswimmers with internal feedback[END_REF]. The clusters are deterministicly prepared by means of sequential capillarity-assisted particle assembly (or sCAPA) [START_REF] Ni | Programmable colloidal molecules from sequential capillarityassisted particle assembly[END_REF]. They furthermore exhibit self-propulsion under spatially uniform AC electric fields in the kHz region, which are applied perpendicularly to the electrodes over which the particles move thanks to asymmetric electrohydrodynamic flows (EHDFs) [START_REF] Ristenpart | Electrohydrodynamic flow around a colloidal particle near an electrode with an oscillating potential[END_REF][START_REF] Ma | Inducing propulsion of colloidal dimers by breaking the symmetry in electrohydrodynamic flow[END_REF][START_REF] Ma | Electric-field-induced assembly and propulsion of chiral colloidal clusters[END_REF][START_REF] Shields | The evolution of active particles: Toward externally powered self-propelling and self-reconfiguring particle systems[END_REF] . The transition between the different dynamical states is triggered by externally-imposed temperature signals, which cause the reconfiguration of the shape and dielectric properties of the clusters. We first begin by conceptually describing our approach to achieve microswimmers with multi-state dynamics and then focus on their experimental realization and characterization.

Self-propelling colloidal finite state machines: the concept

The conceptually simplest case of a self-propelling particle with multiple dynamical states is a two-state particle. Given the requirement for asymmetry, a two-state active particle can be realized by constructing a dumbbell where one of the two lobes changes its properties in response to an input signal.

By selecting EHDF-induced propulsion and temperature signals, dumbbells comprising one non-responsive, e.g. polystyrene (PS), lobe and one responsive, e.g. Poly(N-isopropylacrylamide)-based, lobe fulfill the criteria. Poly(Nisopropylacrylamide) (PNIPAM) is a thermo-responsive polymer that undergoes a reversible lower critical solubility transition at a well-defined temperature. PNIPAM-based particles (or microgels) correspondingly undergo a swelling/deswelling transition at that temperature, called the volume phase transition temperature or VPTT. Upon crossing the VPTT, microgels experience a volumetric change coupled to a variation of their dielectric properties [START_REF] Su | Dielectric relaxations of poly(n-isopropylacrylamide) microgels near the volume phase transition temperature: Impact of cross-linking density distribution on the volume phase transition[END_REF][START_REF] Mohanty | Dielectric spectroscopy of ionic microgel suspensions[END_REF][START_REF] Alvarez | Reconfigurable artificial microswimmers with internal feedback[END_REF][START_REF] Fernández-Nieves | Charge controlled swelling of microgel particles[END_REF]. Self-propulsion generated by EHDFs in dumbbells depends both on the relative dimensions of the two lobes and on their dielectric properties [START_REF] Ma | Inducing propulsion of colloidal dimers by breaking the symmetry in electrohydrodynamic flow[END_REF][START_REF] Ni | Hybrid colloidal microswimmers through sequential capillary assembly[END_REF]. The existence of two different dynamical states for reconfigurable, PNIPAM-based active dumbbells has in fact been recently demonstrated [START_REF] Alvarez | Reconfigurable artificial microswimmers with internal feedback[END_REF].

However, the VPTT of the microgels depends on their chemistry, their internal architecture, e.g. crosslinking density [START_REF] Ruscito | Microgel particles with distinct morphologies and common chemical compositions: A unified descrip-tion of the responsivity to temperature and osmotic stress[END_REF], and the solvent properties [START_REF] Scherzinger | Cononsolvency of poly-n-isopropyl acrylamide (pnipam): Microgels versus linear chains and macrogels[END_REF]. In this work, we synthesize and use two types of thermo-responsive PNIPAM-based microgels to enable the access of multiple dynamical states (see Methods). In particular, we use two different comonomers, methacrylic acid (MAA) and acrylic acid (AA), in the polymerization of PNIPAM to obtain microgels with different VPTTs. The PNIPAM-co-MAA microgels have a green-fluorescent (BODIPY) core and the PNIPAM-co-AA microgels have a red-fluorescent (Nile-red) core, and are henceforth termed green (G) and red (R) microgels. Due to the higher hydrophilicity of AA compared to MAA, its VPTT is shifted to higher temperatures [START_REF] Lung | Preparation, morphology, and thermoresponsive properties of poly(n-isopropylacrylamide)-based copolymer microgels[END_REF][START_REF] Kratz | Influence of charge density on the swelling of colloidal poly(nisopropylacrylamide-co-acrylic acid) microgels[END_REF][START_REF] Gao | Ph-and thermo-responsive poly(n-isopropylacrylamide-co-acrylic acid derivative) copolymers and hydrogels with lcst dependent on ph and alkyl side groups[END_REF]. The VPTT of green-core microgels is T G G ≈ 29 • C and the one of the red-core microgels is T R R ≈ 33 • C (see Supplementary Section 1 for further details).

By using these microgels, we can therefore construct two different types of dumbbells, G-PS and R-PS, respectively, of which we can schematically represent the different states and their transitions as shown in Figure 1A. The G-PS dumbbells undergo a transition between a swollen state G to a collapsed state G (G G ) at a temperature T G G , and correspondingly the transition R R takes place at T R R . As it will be showed in Section 1.2, these two transitions correspond to a switch between two different self-propulsion modes, with a an adaptation of the propulsion velocity

V G V G and V R V R , respectively.
The existence of two different transition temperatures opens the way to the fabrication of microswimmers with three distinct dynamical states by incorporating both G and R microgels in a colloidal trimer together with a PS particle, as schematically shown in Figure 1B. Upon crossing T G G , the trimer goes from a fully swollen state to one where only the G microgel is collapsed GR G R; when T R R is crossed, both microgels collapse and the trimer undergoes the G R G R transition. The order of the transitions is reversed by progressively reducing temperature. In the case of the trimer, because the collapse of one microgel can cause the breaking of the axial symmetry of the cluster, in addition to three distinct translational velocities V GR V G R V G R , three different angular velocities can also be accessed

ω GR ω G R ω G R .

Two-state dynamics: self-propelling responsive dumbbells

After introducing the concept in the previous Section, we now show that the behavior schematically displayed in Figure 1A can be experimentally implemented. We synthesized PNIPAM-co-MAA (G) microgels and PNIPAM-co-AA (R) microgels that have a hydrodynamic diameter of ≈ 2-3 µm before and ≈ 0.7-1.2 µm after their respective VPPT (Figure S4). In both cases the fluorescent cores are made from poly 2-trifluoromethylacrylic (PTFMA) and have a diameter of around 200 nm, to visualize the different microgels under the microscope. The G-PS and R-PS dumbbells are obtained using sCAPA, as previously described [START_REF] Ni | Programmable colloidal molecules from sequential capillarityassisted particle assembly[END_REF][START_REF] Alvarez | Reconfigurable artificial microswimmers with internal feedback[END_REF]. In short, both PS particles and microgels are sequentially deposited into pre-designed traps microfabricated on a polydimethylsiloxane (PDMS) template. An evaporating droplet of the target particle suspension is moved over the PDMS template at a controlled speed and capillary forces selectively place the particles inside the traps. PS particles are deposited first, followed by either the G or R microgels to obtain the dumbbells inside the traps (see Figure 2A-B). After deposition, the particles are linked by thermal sintering at 75 • C for 15 minutes and harvested using an adhesive sacrificial layer of glucose.

The resulting dumbbells are dispersed in a dilute HEPES buffer and confined between two transparent electrodes consisting of glass slides coated by a Cr and a Au layer covered with a polyethylene glycol (PEG) brush (see Methods). The electrodes are separated by a 240 µm spacer and connected to a function generator that applies an AC electric field of 7 V peak-peak amplitude ( 300 V cm -1 ) and 800 Hz frequency perpendicular to the electrodes. Upon application of the AC field, both the electrodes and the particles polarize; the presence of the particles close to the bottom electrode (where they sediment) causes a local distortion of the electric field generating a component that is tangential to the substrate, which in turns leads to the recirculation of ions, the so-called electrohydrodynamic flows (EHDFs), around each colloid [START_REF] Ristenpart | Electrohydrodynamic flow around a colloidal particle near an electrode with an oscillating potential[END_REF]. At a fixed voltage and frequency, the magnitude of the EHDFs depends on the particle size and dielectric properties [START_REF] Ni | Hybrid colloidal microswimmers through sequential capillary assembly[END_REF][START_REF] Ma | Inducing propulsion of colloidal dimers by breaking the symmetry in electrohydrodynamic flow[END_REF][START_REF] Ristenpart | Electrohydrodynamic flow around a colloidal particle near an electrode with an oscillating potential[END_REF]. However, for homogeneous spherical particles, the EHDFs are radially symmetric and do not induce any net propulsion. Conversely, for dumbbells with lobes of different size and dielectric properties, the symmetry of the EHDFs is broken and a net fluid flow is generated, which leads to self-propulsion. In particular, if U i is the magnitude of the velocity of the EHDF generated around particle i, the propulsion velocity of the dumbbell can be written, in first approximation, as v ij = U i r j + U j r i /(r i + r j ), where r i is the radius of particle i (or j, respectively) [START_REF] Ma | Inducing propulsion of colloidal dimers by breaking the symmetry in electrohydrodynamic flow[END_REF]. Given that both U P S and r P S do not strongly depend on temperature, a sudden change in propulsion velocity is expected upon crossing T G G and T R R for the G-PS and R-PS dumbbells, respectively, because both the size and the dielectric properties of the microgels switch between different states upon crossing their respective VPTT (see Supplementary Section 2 for further details).

Experimentally, we induce controlled and rapid temperature variations by exploiting the plasmonic heating of the Au layer within the electrode upon illumination at 395 nm and 555 nm [START_REF] Ho Son | Ultrafast photonic pcr[END_REF]. By adjusting the light intensity, we can locally heat the system above T G G or T R R and correspondingly reconfigure the dumbbells (2A-B; (see Supplementary Section 3 for further details) for an experimental calibration and a numerical verification of the heating via finite element modelling (FEM) with COMSOL Multiphysics [33]). The system equilibrates within seconds and cools down upon removal of the illumination to ensure the reversible transition between the different states.

In line with previous observations [START_REF] Alvarez | Reconfigurable artificial microswimmers with internal feedback[END_REF], the collapse of the microgels significantly reduces the magnitude of the propulsion velocity of the microgel-polystyrene dumbbells (Figure 2C). In particular, T G G or T R R are sufficiently separated so that G microgels collapse while R ones are still in the swollen state, resulting in reconfiguration and velocity changes at distinct temperatures. This response follows the scheme proposed in 1A and holds the key to access additional dynamical states when combining both microgels in one active cluster.

Assembly of multi-microgel colloidal clusters

However, in order to realize the system proposed in Figure 1B, we need to achieve deterministic control on the relative position of the two microgels in the active G-R-PS trimer. As it can be seen in 2A-B, conventional sCAPA in rectangular traps does not have sufficient positional control for the microgels, which are deposited on either side of the PS particles.

In order to overcome this limitation, we have recently demonstrated a further refinement of sCAPA that exploits for further details). [START_REF] Steven Van Kesteren | Printing on particles: combining two-photon nanolithography and capillary assembly to fabricate multi-material microstructures[END_REF] We use the different height of the different sections of the trap to guide the deposition of the polystyrene particles and the R and G microgels into trimers with well-defined opening angles. The assembly process requires careful tuning of the trap geometry as well as of the deposition medium, speed, direction and order [START_REF] Ni | Insights into mechanisms of capillary assembly[END_REF]. In particular, the PS particles are deposited first, followed by the R and then the G microgels (see Figure 3B). By changing the geometry of the traps, we produced three-particle clusters with opening angles of 60, 120 and 180 • degrees. We refer to these systems as 60-, 120-, or 180-clusters and examples are shown in Figure 3C. The deposition yields go up to 75% for the 120-clusters (see Supplementary Section 5 for further details) and we use the same linking and harvesting procedures as for the dumbbells.

Three-state motility of 120-clusters

We first focus on the dynamical response of the 120-clusters. After assembly and harvesting, the trimers are inserted in the same experimental cell that we previously described for the dumbbells and the transverse AC electric field (7 V peak-to-peak at 800Hz) is applied together with illumination of varying power density I at 395 nm to induce the local heating (Figure 4A). In particular, we use a sequence of illumination/heating steps in the following order: 30 s no illumination (≈ 21 • C), 60 s at 33 mW mm -2 (≈ 31 • C), 30 s at 92 mW mm -2 (≈ 45 • C) and finally 60 s no illumination (Figure 4B). This sequence enables us to switch between a state where both microgels are swollen (GR), one where one microgel is swollen and the other one is collapsed (G'R), one where both microgels are collapsed (G'R') and back to the initial state where both microgels are swollen again (GR) (Figure 4B). We observe rapid, reversible switching amongst the various states, which is reflected in a qualitative and quantitative variation of the trajectories (Figure 4C): in state (GR) the trajectory is curvilinear with a constant radius of ≈ 12µ m, in (G'R) the particle continues with a strong chiral motion but the radius of curvature is significantly decreased to 5.5 µm, and, finally, in (G'R') the propulsion velocity drops and the rotational diffusion increases, resulting in less-directed motion. Upon removal of fluorescence illumination, the trajectory resumes the original shape of state (GR).

By quantitatively tracking the motion of the trimers as a function of time/illumination, we further characterize their motility in terms of their instantaneous and angular velocity. The two quantities are coupled due to the nature of the trajectory (Figure 4D), but they can be independently measured from the experiments (see Supplementary Section 6 for the calculation of the angular velocity) and both contribute to how the active particles explore space (see Figure 5). The shape of the trajectories and net swimming velocity are rationalized by considering the direction of the propulsive force and of viscous drag. As we have previously discussed in Figure 1, in the case of dumbbells, propulsion is aligned with the direction of compositional asymmetry and, as a consequence of geometry, the center of propulsion and of hydrodynamic drag coincide. However, this in not necessarily the case for the trimers. As schematically shown in Figure S12 (Supplementary Section 7), we can rationalize the propulsion of the trimer as the linear superposition of the propulsion generated by two dumbbells, respectively connecting the PS particles to the G and the R microgels.

Variations in the magnitude of each velocity vector and of the size of the respective dumbbell lobes, may cause a misalignment between the center of propulsion and the center of hydrodynamic drag, giving rise to a rotation-translation coupling and the corresponding emergence of a active torque [START_REF] Brenner | Coupling between the translational and rotational brownian motions of rigid particles of arbitrary shape ii. general theory[END_REF][START_REF] Chakrabarty | Brownian motion of arbitrarily shaped particles in two dimensions[END_REF][START_REF] Tong Zhang | Diffusion tensors of arbitrary-shaped nanoparticles in fluid by molecular dynamics simulation[END_REF][START_REF] Kraft | Brownian motion and the hydrodynamic friction tensor for colloidal particles of complex shape[END_REF]. With this simple image in mind, and considering the temperature response of each individual self-propelled dumbbell, in state (GR), each microgel has a similar size and generates similar EHDFs. However, these are not identical, leading to a breaking of the in-plane symmetry of the trimer's propulsion and drag, which leads to a curvilinear trajectory with a well-defined chirality and a large radius. In (G'R), the G-microgel collapses with a dramatic drop on its propulsion velocity, while the propulsion velocity of the R-microgel increases slightly. The greater asymmetry between both microgels leads to an increased torque, and correspondingly to a tightening of the curvilinear radius. The G'R-trimer's instantaneous velocity is slightly larger compared to the one in state (GR), due to the moderate increase of the EHDFs of the R-PS dumbbell, in conjunction with the reduced drag stemming from the collapse of the G-microgel. The combination of these factors leads to an increased angular velocity. Finally, in (G'R'), both microgels are collapsed and provide a significantly lower propulsive force. Correspondingly, the trimer's velocity dramatically drops. This fact, combined with the overall decreased size of the microswimmer implies that rotational diffusion plays a greater role in determining the trimer's trajectory in state (G'R').

Multi-state space exploration

The same simplified approach can be used to describe the motility of trimers with the other internal angles (see Supplementary Section 7 for further details), where reconfiguration can be used to tune how different active clusters explore space. By examining active trajectories, we immediately observe a clear qualitative difference in the dynamical behavior amongst the 60-, 120-and 180-clusters (Figure 5A).

The 60-clusters (Figure 5A top) have the largest instantaneous velocity in state (GR). However, unlike the 120-clusters, their velocity immediately drops in (G'R) and remains similar in (G'R'). The 180-clusters (Figure 5A bottom) conversely have a low instantaneous velocity in state (GR) compared to the 120-cluster. However, the transition to state (G'R)

shows a much more significant speeding up without a large change in the angular velocity, while in state (G'R') the 180-cluster behave very similar to the 60-and 120-clusters. We therefore show that simple reconfiguration strongly affects the motility of the clusters in the different states.

Similarly to our description of the 120-cluster, the behaviour of the 180-cluster can be rationalized by the force vector model shown in the Supplementary Section 7. In state (GR) the propulsive forces of the 2 microgel-PS dumbbells are similar in magnitude but oppose each other leading to a very small net propulsion. Switching to (G'R) breaks this balance, the G microgel collapses and the clusters speed up. However, the behaviour of 60-clusters appear different, and self-propulsion essentially ceases already at T G G and does not change upon crossing T R R . We hypothesize that, given their dimensions relative to the PS particle, the two microgels substantially overlap in the swollen state with an internal angle of 60 • , such that the collapse of the G microgel already causes the R microgel to collapse, practically eliminating any difference between (G'R) and (G'R') (see Supplementary Section 8 for further details). .

Finally, the three different states of the clusters are associated with different ways to explore space, which can be represented by plotting their mean-square displacements (MSDs) [START_REF] Archer | Glancing angle metal evaporation synthesis of catalytic swimming janus colloids with well defined angular velocity[END_REF]. The 60-and 180-cluster have a clear 'fast' state, which is (GR) for the 60-clusters and (G'R) for 180-clusters. These states correspond to high instantaneous velocities and small angular velocities explore, for which space is explored significantly faster by particle ensembles than the low-velocity states at any time. The MSDs for 120-cluster are more nuanced, where mean displacements are greater in state (G'R) at short times, but motion becomes more confined at longer times due to the strong chiral motion. The distinct dynamical behavior of different clusters in different states indicates a broad range of possibilities to develop adaptive artificial microswimmers by design.

Conclusions and outlook

In this manuscript we have demonstrated that the combination of deterministic assembly using sCAPA and the utilization of stimuli-responsive microgels enables the realization of synthetic microswimmers that can transition among distinct internal states, which are in turn coupled to their dynamics. Importantly, and differently from typical light-or magnetically-activated particles, where the stimuli for propulsion and the ones that regulate motility coincide [START_REF] Koohee Han | Sequence-encoded colloidal origami and microbot assemblies from patchy magnetic cubes[END_REF][START_REF] Lozano | Phototaxis of synthetic microswimmers in optical landscapes[END_REF][START_REF] Lozano | Diffusing wave paradox of phototactic particles in traveling light pulses[END_REF], our system's propulsion mechanism is orthogonal to the reconfiguration one.

The current study focuses on demonstrating the feasibility of self-propelling finite state machines at the colloidal scale and on the characterization of the single-particle dynamics for different microswmimmer designs. However, the independence of the reconfiguration pathway enables robust control of the particle motility and the possibility to easily implement spatially and temporally-modulated motility landscapes to investigate collective effects, in analogy with investigations of the collective dynamics of light-regulated bacteria [START_REF] Arlt | Painting with light-powered bacteria[END_REF][START_REF] Frangipane | Dynamic density shaping of photokinetic E. coli[END_REF]. In living organisms, the adaptation to external stimuli is internally regulated and characterized by finite transition times. Conversely, synthetic microswimmers subjected to sophisticated perception and adaptation schemes [START_REF] François | Group formation and cohesion of active particles with visual perception-dependent motility[END_REF][START_REF] Khadka | Active particles bound by information flows[END_REF][START_REF] Fränzl | Active particle feedback control with a single-shot detection convolutional neural network[END_REF] are externally controlled and the feedback between input signals and motility regulation can be considered instantaneous over the characteristic timescales of active motion. Nonetheless, the presence of finite response times, even for external control strategies, has been shown to lead to emergent dynamical and collective effects [START_REF] Angel Fernandez-Rodriguez | Feedback-controlled active brownian colloids with space-dependent rotational dynamics[END_REF][START_REF] Yang | Optimal navigation of self-propelled colloids[END_REF][START_REF] Mijalkov | Engineering sensorial delay to control phototaxis and emergent collective behaviors[END_REF]. Our colloidal FSMs fall in between these categories, where propulsion and reconfiguration are externally triggered, but where the transitions between the different states are encoded during fabrication and take place with a characteristic internal time scale.

Given our findings, many further avenues for future work open up. For instance, the level of single-particle control may already be amenable to the demonstration of lab-on-chip devices. The AC-electrodes could be easily integrated in microfluidic systems, where, guided by thermal signals, the reconfigurable microswimmers can navigate and perform different functions. For example, we envisage that the 60-clusters accumulate in regions where the temperature is between 29-33 • C, corresponding to state (G'R), where they have a small MSD. They could therefore efficiently explore space in state GR until they reach a region with a higher temperature and linger there, i.e. to aid or perform a chemical reaction, if carrying the appropriate load. Conversely, in the same state, 180-clusters would spend little time in these hotter regions due to a corresponding increase in their MSDs. They could thus be used to remove reaction products and transfer them to colder regions, where their motility drops. Even though thermal signaling may not be practical for all applications, incorporating responses to different stimuli may offer further options.

In fact, the modular assembly enabled by sCAPA makes it possible to incorporate other soft components, which can reconfigure in response to different stimuli. PNIPAM-based microgels that are responsive to light [START_REF] Das | Microgels loaded with gold nanorods: Photothermally triggered volume transitions under physiological conditions[END_REF], pH [START_REF] Kratz | Influence of charge density on the swelling of colloidal poly(nisopropylacrylamide-co-acrylic acid) microgels[END_REF][START_REF] Gao | Ph-and thermo-responsive poly(n-isopropylacrylamide-co-acrylic acid derivative) copolymers and hydrogels with lcst dependent on ph and alkyl side groups[END_REF], redox state [START_REF] Suzuki | Temporal control of self-oscillation for microgels by cross-linking network structure temporal control of self-oscillation for microgels by cross-linking network structure[END_REF], magnetic fields [START_REF] Rittikulsittichai | Multi-responsive hybrid particles: Thermo-, ph-, photo-, and magneto-responsive magnetic hydrogel cores with gold nanorod optical triggers[END_REF] and (bio)chemical molecules [START_REF] Tang | Contraction-type glucose-sensitive microgel functionalized with a 2-substituted phenylboronic acid ligand[END_REF] have been synthesized. The use of multiple, orthogonal stimuli would greatly increases the number of possible states, compared to using only one stimulus, e.g. temperature, with different transitions.

Finally, in order to successfully assemble precisely-programmed reconfigurable clusters, we had to push the boundaries of sCAPA beyond the state of the art with the introduction of 3D height profiles within the traps to direct the deposition of soft and hard particles in deterministic shapes. In the future, we also envisage that additional strategies can be used to incorporate reconfigurable parts in microscale active units. Recent advances in 3D-printing of microscale responsive hydrogels with two-photon nanolithography could allow for more flexible fabrication of soft structures [START_REF] Hippler | Controlling the shape of 3d microstructures by temperature and light[END_REF], toward hybrid colloidal materials [START_REF] Hu | Magnetic soft micromachines made of linked microactuator networks[END_REF]. All of these efforts will take a very active community of researchers and engineers one step, or more, closer to the goal of realizing autonomous microscale systems with capabilities mirroring the ones of larger scale robotic units.

Materials and Methods

All chemical if not state otherwise were used as provide by the supplier. Triton X-45, Acrylic acid (AA),Methacrylic Acid (MAA), N, N' Methylenebis(acrylamide) (BIS), 2,2,2-Trifluoroethyl methacrylate (TFMA), Sodium dodecyl sulfate (SDS), Potassium persulfate (KPS), BODIPY, and Nile red were purchased from Sigma-Aldrich. N-Isopropylacrylamide (NIPAM) was purchased from Sigma-Aldrich and purified by recrystallization in Toluene/Hexane 50:50. 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) was purchased from VWR.

Synthesis of core-shell pNIPAM microgels

The microgels with fluorescent cores and thermo-responsive shells were prepared in a two-step synthesis [START_REF] Go | Programmable co-assembly of oppositely charged microgels[END_REF]: first the PTFMA-cores containing a fluorescent dyes were prepared using free radical emulsion polymerization, then a shell of PNIPAM and PAA or PMAA co-polymer was grown by free radical precipitation polymerization around this core. For the cores, TFMA (10 mL; 12 g; 70,3 mmol), NIPAM (940 mg; 8,31 mmol), SDS (30 mg; 0,104 mmol) and Nile red (5 mg; 0.016 mmol) or BODIPY 493/503 (5 mg; 0,016 mmol) were added in water (30 mL) and stirred at 600 rpm for 20 min and purged with N 2 . The reaction mixture was heated up to 70 • C. KPS (25 mg; 0.093 mmol), previously dissolved in water (2,5 mL), was added after the emulsion was stirred for 15 min at 70 • C. After 4 h of stirring at 70

• C the reaction was stopped by contact with air. The resulting particles were directly filtered. The purification of the particles was carried out via centrifugation followed by decantation, addition of Milli-Q water and particle redispersion. The purification via centrifugation was performed three times. To remove SDS residues, the redispersed particles were dialysed in water for 24 -72 h (dialysis tube membrane: 12 -14 kDa). The particles were analysed with dynamic light scattering (ZetaSizer Nano DLS).

The PNIPAM-co-AA / PNIPAM-co-MAA shells were grown by first dissolving NIPAM(1.15g), BIS(11.5 mg) and MAA or AA(88.5 µl) in 50 ml milli-Q water. Then 300 µl of the core particle suspension was added. The mixture was stirred at 600 rpm for 20 min and purged with N 2 . The reaction mixture was heated up to 70 • C and KPS (25 mg; 0.093 mmol) was slowly added. After 2 h, the reaction reached completion and the microgel suspension was filtered and dialysed for 48h (dialysis tube membrane: 12 -14 kDa). The particles were characterized with DLS.

Fabrication of colloidal molecules

The responsive microswimmers were prepared using sequential capillarity-assisted particles assembly (sCAPA). The basic principles of sCAPA are described by Ni et al. [START_REF] Ni | Programmable colloidal molecules from sequential capillarityassisted particle assembly[END_REF]. Here, we used an adapted version of sCAPA, which uses masters prepared with direct-writing via two-photon nanolithography, as opposed to Si masters made with conventional lithography methods. Briefly, negative masters were prepared with two-photon lithography (Nanoscribe Photonic Professional GT2) using an 63x oil-immersion objective and IP-Dip solution set. [START_REF] Steven Van Kesteren | Printing on particles: combining two-photon nanolithography and capillary assembly to fabricate multi-material microstructures[END_REF] The masters contained 6 smaller areas with 10,000 "traps" each, and one area was used to prepare the particles for one experiment. After printing and developing under standard conditions, these masters were post-cured for 2h under a UV-lamp and coated with a perfluorosilane using chemical vapor deposition. PDMS (Sylgard 184) was used to make templates from these masters. sCAPA was performed as described in the following paper [START_REF] Ni | Insights into mechanisms of capillary assembly[END_REF]. The PS particles were deposited from a 0.035% Triton X-45 and 3.5 mM SDS solution in Milli-Q water. The microgels from a 0.04% Triton X-45 in 1 mM HEPES pH 7.4 or 0.025% Triton X-45 in 1 mM MES pH 4.5 for green and red-core microgels, respectively. All deposition were carried out at 25 • C with a deposition rate of 3 µm s -1 .

Active motion experiments

The colloidal clusters were activated using AC electric fields in a closed cell with the top and bottom electrodes consisting of transparent conductive slides separated by a 240 µm-thick spacer (Grace Bio-Labs SecureSeal, USA, custom shape). The conductive slides were boro-silicate microscope slides coated with 3 nm Cr and 10 nm Au by thermal evaporation. The bottom slide on which the particles move was treated with thiolated polyethylene glycol (6 kDA, 1 mM in Milli-Q water, 30 min) to prevent sticking. The colloidal molecules were transferred from the sCAPA-templates to the top electrode, previously coated by a 3 µm-thick spin-coated layer of glucose (40% wt. in DI-water, 30 s 4000 rpm). Upon exposing the top slide to the suspending medium (0.1 mM HEPES buffer at pH 7 and 0.001% wt. Triton in Milli-Q water) and closing the cell, the glucose layer dissolved and the particles sedimented to the bottom electrode. The conductive slides were connected to function generator (Agilent 33500B) with copper leads made from 50 µm copper foil and the experiments were preformed on a Eclipse Nikon Ti2-e with a Lumencor Spectra II light box and a 20x objective. The particles were actuated by applying an AC field with 800 Hz frequency and 7 V peak-to-peak amplitude. The field was applied for 15 min to equilibrate the sample before collecting data. Movies were recorded in a 1024*1024 px 2 window (340*340 µm 2 ) at 10 frames per second. The local temperature was regulated through illumination with the 395 nm and 555 nm light sources of the light box (see SI for more details).

All the particles in the field of view were located and tracked using the TrackPy library in Python [START_REF] Daniel | softmatter/trackpy: Trackpy v0[END_REF]. Stuck particles and particles that get stuck during the experiment were omitted from the analysis. The instantaneous velocity of every individual particle was determined by measuring their displacements over 0.5 s windows (5 frames). The angular velocity was determined by a least-square fit the auto-correlation function of the direction of motion over 30 s windows (see SI for more details), corresponding to the time windows over which temperature was changed.

Figure 1 :

 1 Figure 1: Schematic representation of the FSM model for reconfigurable colloidal clusters. A, Representation of the reconfiguration of the of G-PS and R-PS dumbbells upon crossing the respective transition temperatures T G G and T R R and schematic of the corresponding transition between the different states as a function of temperature T . B, Extension of the previous concept to a three-state particle (R-G-PS trimer).

Figure 2 :

 2 Figure 2: Two-state microswimmers. A-B, Combined bright-field+fluorescence micrographs of G-PS (A) and R-PS (B) dumbbells in the sCAPA traps. The scale bars are 5 µm. C-D, Schematic of the experiment for the two-state temperature-induced switching in the self-propulsion of G-PS (C) and R-PS (D) dumbbells. The curved arrows indicate the EHDFs generated by the PS and microgel lobes, respectively, and the straight arrows represent the net propulsion due to these EHDFs. Plasmonic heating of the substrate illuminated with a light of intensity I G G (I R R ) raises the local temperature above T G G (T R R ) inducing a phase transition in the microgels. Note that because T R R > T G G , the corresponding illumination intensity is higher. E, Mean instantaneous self-propulsion velocity < v > as a function of temperature T for the R-PS (triangles) and G-PS (squares) dumbbells. The shaded bands are the 99% confidence intervals calculated over 350 and 70 particles for the R-PS and G-PS, respectively . The vertical dashed lines mark the transition temperatures for the green and red core microgels, respectively. The inset schematically shows the switching of the self-propulsion state upon crossing the critical temperature, taking a R-PS dumbbell as an example. The curved arrows represent the EHDFs generated by the microgel and the PS particle, with characteristic velocities U µgel and U PS , respectively, and the straight arrows indicate the net propulsion with velocity V.

Figure 3 :

 3 Figure 3: Assembly of reconfigurable trimers. A, Schematic showing the filling of the 3D-traps with sCAPA for a single colloidal 120-cluster. The numbers indicate the order of the deposition steps and the arrows respective deposition directions. B-D, Combined bright-field+fluorescence micrographs of R-G-PS trimers in the sCAPA traps with 60 (B), 120 (C) and 180 (D) degrees between the microgels. Scale bars are 5 µm

Figure 4 :

 4 Figure 4: Three-state dynamics for G-R-PS 120-clusters. A, Schematic illustrating the 120-cluster in state (GR), (G'R) ,or (G'R') depending on the illumination conditions. The arrows indicate the EHDFs and the net propulsion like in the previous figures. B, Light exposure sequence (dashed line) over time and resulting temperatures (solid line) as predicted by finite elements simulations. The color coding of the temperature corresponds to the color scale bar on the right of the graph. C, Example trajectory of a G-R-PS 120-cluster color-coded with time and corresponding temperature as represented by the color scale bars on the left of the graph. The scale bar is 20 µm. D, Mean instantaneous velocity (solid line) and angular velocity (symbols) of the microswimmer over time color-coded with temperature. The shaded bands and vertical error bars are the 99% confidence intervals calculated over 39 trajectories for the instantaneous velocity and angular velocity, respectively.

Figure 5 :

 5 Figure 5: Multi-state motility of different active clusters. Each row corresponds to the 60-, 120-or 180-clusters, respectively. A, Schematic of the representative motion of the three clusters in state (GR),(G'R) and (G'R'). B, Example trajectories color-coded with temperature (same temperature range as in Figure3). Scale bars are 20 µm . C, Scatter plot of the mean instantaneous velocity and angular velocity for each particle in (GR),(G'R) and (G'R'). The elliptical shaded regions correspond to one standard deviation of both quantities calculated over 54, 39 and 45 trajectories for the 60-, 120-or 180-clusters, respectively. D, Ensemble-averaged MSD in (GR),(G'R) and (G'R'). Symbols: experimental data. Dashed lines: MSD fits for a rotating active Brownian particle[START_REF] Archer | Glancing angle metal evaporation synthesis of catalytic swimming janus colloids with well defined angular velocity[END_REF].

Acknowledgements

LA acknowledges financial support from the European Soft Matter Infrastructure (EUSMI) proposal number S180600105. This project has received funding from the European Research Council (ERC) under the the European Union's Horizon 2020 research and innovation programme grant agreement No 101001514. We thank Alexander Kuehne and Dirk Rommels for help with particle synthesis and discussion.


Franck Komi Gbekou
email: komi.gbekou@univ-eiffel.fr

Karim Benzarti
email: karim.benzarti@univ-eiffel.fr

Abderrahim Boudenne

Experimental study on cement mortars including micronized miscanthus fibers NoMaD 2022

Keywords: Micronized miscanthus fibers, Cement mortar, Thermal and hygric properties, Mechanical strength, Microstructure

This study investigates the improvement of thermal and hygric performances of cement mortars through the introduction of micronized miscanthus fibers. Micronized fibers were chosen because they are expected to promote fine and homogeneous fiber distribution within the cementitious matrix, hence opening possibilities for the design of 3D printable mortar mixes including vegetal fibers.

An experimental protocol was first developed for preparing five mortar mixes with miscanthus fiber contents up to 7 wt.%. These bio-based mortars were then characterized at the age of 28 days, in terms of mechanical strength (under flexural and compression tests), thermophysical properties (determination of the thermal conductivity/diffusivity and the volumetric Heat Capacity by the Hot-Disk method), and in terms of hygric properties as well (evaluation of the Moisture Buffer Value MBV according to the Nordtest method). The results of this experimental campaign showed that increasing the miscanthus fiber content leads to large improvements in both thermal resistance (up to 87%) and moisture buffer capacity of mortars (with MBV values up to 2.05), suggesting that such bio-based mortar is a good insulating material and has an excellent ability to mitigate external moisture variations. On the other hand, the introduction of vegetal fibers was also found to decrease very significantly the mechanical strength of the modified mortars, making these latter incompatibles with structural applications. Nevertheless, the developed bio-based mortars retain sufficient mechanical properties for handling and are suitable for building insulation.

I. INTRODUCTION

World energy consumption is growing rapidly in the construction sector, causing substantial increase in CO2 emission. This trend is mainly driven by economic development and population growth, and contributes to exhaust natural resource and degrade the environment. Therefore, current public policies promote the development of innovative technologies capable of improving energy efficiency, with specific interest in "green technologies". In this context, the replacement of conventional building materials by bio-based alternatives with low thermal conductivity has emerged as an interesting solution for upgrading thermal performance and reducing carbon footprint of buildings. In addition, these bio-based materials generally exhibit hygroscopic behavior which enable them to absorb/release moisture, and hence, to reduce humidity variations of the indoor air. Therefore, they are not only effective as thermal insulation materials, but also offer benefits in terms of comfort and health for the occupants.

Numerous studies have explored the use of vegetable fibers (hemp, flax, cork, rice husk, date palm fibers, ...) in the formulation of ultra-lightweight concrete intended for the insulation of buildings [START_REF] Ashour | The influence of natural reinforcement fibres on insulation values of earth plaster for straw bale buildings[END_REF][START_REF] Chen | Acoustic performance and microstructural analysis of bio-based lightweight concrete containing miscanthus[END_REF][START_REF] Chikhi | Experimental investigation of new biocomposite with low cost for thermal insulation[END_REF][START_REF] Liu | Development of lightweight concrete with high resistance to water and chloride-ion penetration[END_REF][START_REF] Panesar | The mechanical, transport and thermal properties of mortar and concrete containing waste cork[END_REF][START_REF] Haba | Hygric properties and thermal conductivity of a new insulation material for building based on date palm concrete[END_REF][START_REF] Chennouf | Hygrothermal characterization of a new bio-based construction material: Concrete reinforced with date palm fibers[END_REF]. Most authors have reported that the incorporation of vegetal fibers into cement materials leads to significant improvement in the thermal performance (in particular a large reduction of the thermal conductivity) and provides superior moisture absorption capacity. On the other hand, mechanical properties can be severely degraded. These effects are generally correlated to the microstructure of these bio-based composites, which contains a high density of macro and micro-porosities.

Among the available vegetal fibers, miscanthus fibers have gained interest over the last decade. Miscanthus is a perennial plant with a high potential for biomass production and whose exploitation does not compete with human/animal feeding [START_REF] Dias | Optimisation of the mechanical properties of Miscanthus lightweight concrete[END_REF][START_REF] Lewandowski | 2-Miscanthus[END_REF][START_REF] Lewandowski | Delayed harvest of miscanthus-influences on biomass quantity and quality and environmental impacts of energy production[END_REF]. Miscanthus can be cultivated over a period of about 15 years, adapts easily to different climates and soil conditions, and has low requirement in terms of fertilization or pesticide inputs. In France, this crop is mainly cultivated in the northern regions of the country, and an effective valorization chain from the farm to the industry is currently being organized. However, the use of miscanthus fibers in building materials is still poorly documented in the literature and further research is needed in this field.

In this context, this study explores the possibility of incorporating micronized miscanthus fibers into a cement mortar and investigates the effect of this incorporation on the properties of the resulting bio-based composite. The fibers are micronized to allow a finer and more homogeneous distribution in the mortar matrix, with a view to developing 3D printable materials including vegetal fibers in a next part of the project. The experimental program involves the preparation of mortar specimens with various dosages of miscanthus fibers in the range 0-7 wt.%. Multiphysics characterization is then carried-out on these specimens to assess the effects of the fiber addition.

II. MATERIALS AND METHODS

A. Materials and sample preparation

The vegetal fibers selected for this study are micronized miscanthus fibers with diameters in range 200-500 µm, and conditioned in the form of a fine powder. This product was supplied by Addiplast Group (Saint-Pal-de-Mons, France) and it was used as-received. Figure 1 provides typical pictures of these fibers at various magnifications, as obtained by scanning electron microscopy (SEM), revealing their porous microstructure.

Regarding the reference cement mortar, an existing formulation from the literature was chosen [START_REF] Khalil | Formulation et caractérisation chimique et rhéologique des mortiers imprimables en 3D à base de mélanges de ciments Portland et sulfoalumineux[END_REF], which was initially developed for additive manufacturing applications. The mix design contains a Portland cement EXTREMAT® CEM I 52.5 N (noted CEM I), a fast-setting Sulfo-aluminous cement Alpenat R² (noted CSA), both manufactured by VICAT Company (L'Isled'Abeau, France), a superplasticizer (SP) SIKA VISCOCRETE TEMPO 11 and standardized siliceous sand (maximal size 2 mm).

The experimental study was carried out on 5 different mortars including the reference one. Mortar mixes denoted Mxx were designed with different dosages xx of dry fibers ranging from 0 to 10 wt. % (with respect to the total mass of all components of the reference mortar M0). The different mortar formulations are summarized in Table 1. The effective mass fractions of miscanthus fibers contained in the fresh mixes are also given in the table (these values are obtained by considering the total amount of water in the mixes). The corresponding volume fractions are difficult to evaluate, because the intrinsic density of the micronized miscanthus fibers is not precisely known; therefore, they are not mentioned in the table.

The reason for the reported increase in the water content of mortars with the added fiber content is explained in the next paragraph. Sample preparation was carried out with a programmable mortar mixer, following specifications of EN 196-1 standard. The mixing procedure of the reference mortar M0 includes several steps:

1. First, the mixture of the two types of cement (OPC and CSA in the proportion 97/3 wt. %) was placed in the mixer tank. The water containing superplasticizer was then added to the cement and the zero time was counted from the moment the water came into contact with the cement, 2. The mixing started at a low speed and the sand was added slowly during the first minute while mixing at low-speed. The mortar was then mixed at high speed for 30 more seconds; 3. Rotation was stopped for 30 seconds, to scrape the mortar from the bottom of the tank; 4. Another high-speed mixing was finally performed for 2 minutes 30 seconds.

Regarding mortars with miscanthus fibers, a similar mixing protocol was used but preliminary conditioning of the fibers was necessary. Preliminary tests showed indeed that the addition of dry fibers into the fresh mix leads to a rapid absorption of the mixing water by the miscanthus fibers, which degrades considerably the workability of the system. This can be attributed to the high water absorptivity of these vegetal fibers, which is reported between 390-500% depending on the size of the fibers [START_REF] Chen | Acoustic performance and microstructural analysis of bio-based lightweight concrete containing miscanthus[END_REF]. After those preliminary tests and based on a literature review [START_REF] Onuaguluchi | Plant-based natural fibre reinforced cement composites: A review[END_REF], it was decided to pre-soak the fibers in water for 2 hours before incorporation into mortars. The soaked fibers were then placed in a tissue to eliminate the excess water, and they were added to the fresh mix at the end of step 4. An additional low-speed mixing was performed for 30 seconds followed by high-speed mixing for 1 minute to ensure homogenous fiber distribution.

after mixing, the various mortar mixtures were cast into metallic molds, to prepare parallelepiped samples of dimensions 4×4×16 cm 3 dedicated to mechanical characterizations, as well as 4x4x8 cm 3 samples dedicated to thermal and hygric characterizations. The molds were filled in 2 successive layers, and each layer was subjected to 52 blows (for about 1 min) using a shock table. After casting, the samples were covered and stored for 24 h at (20 ± 1) °C. They were demolded 24 hours later, hermetically wrapped with a plastic film, and finally conditioned at (20 ± 1) °C until the mechanical characterizations at 28 days. After removing the plastic film at 28 days, samples dedicated to thermophysical and hygric characterizations were additionally stored for few days at 23°C and 50% relative humidity (RH) to reach an equilibrium state before testing.

B. Characterization techniques

The water-accessible porosity and the apparent density of the mortars were determined according to NF P18-459 standard. Specimens of dimension 4x4x8 cm 3 were cut into smaller samples (2x2x4 cm 3 ) and followed all the steps of characterization described in the standard.

The mechanical strength of the mortars (under flexural and compression tests) was determined at 28 days using a 300 kN capacity press and according to the NF EN 196-1 standard. For each mortar, 6 samples were tested under three-point bending. The resulting 12 halves specimens were then tested in compression. In both cases, mean values and standard deviations were determined.

The thermal conductivity of the various mortars was determined by the Hot Disk (HD) method based on the transient plane source theory (ISO 22007-2), using a TPS 2500S apparatus. This method enables a fast, accurate, and non-destructive evaluation of the thermal conductivity (λ in W.m -1 .K -1 ), the thermal diffusivity (α in m 2 .s -1 ), and the volumetric heat capacity (ρ.Cp in MJ.m -3 .K - 1 ). Experiments were carried out in a climatic chamber, at a constant relative humidity of 50% and over a temperature interval from 5°C to 45°C.

The Moisture Buffer Value (MBV) indicates the capacity of a material to store/release moisture when exposed to moisture variations. MBV determination was carried out for the various mortars based on the NORDTEST Protocol [START_REF] Rode | Moisture buffering of building materials[END_REF]. This protocol consists in exposing the sample to a constant temperature, with a cyclic variation in relative humidity between high (75% RH) and low (33% RH) levels for periods of 8 and 16 hours respectively.

III. RESULTS AND DISCUSSION

A. Workability of the fresh mortar mixes

In order to evaluate the workability of the various mortar formulations, the slump of the fresh mixes was determined immediately after mixing using the Abrams mini-cone method. Figure 2 shows the evolution curve of the slump versus the weight fraction of miscanthus fibers in the mortars. As a general trend, the mortar workability is significantly reduced when the fiber dosage increases. This effect can not be attributed to the absorption of water by the fibers since these latter were already saturated before incorporation, but it may result from a modification of the overall granular distribution within the material. The same trend was observed by several authors with various types of fibers. For example, Chen et al. observed a similar decrease in the workability of ultra-lightweight concrete with the addition of miscanthus fibers [START_REF] Chen | Acoustic performance and microstructural analysis of bio-based lightweight concrete containing miscanthus[END_REF][START_REF] Chen | Bio-based ultra-lightweight concrete applying miscanthus fibers: Acoustic absorption and thermal insulation[END_REF]. Savastano et al. and Panesar & Shindman observed the same trend for mortars including sisal fibers and waste cork, respectively [START_REF] Panesar | The mechanical, transport and thermal properties of mortar and concrete containing waste cork[END_REF][START_REF] Savastano | Plant fibre reinforced cement components for roofing[END_REF]. The apparent density of mortars decreases almost linearly as the fiber dosage increases, whereas the opposite trend is observed for the evolution of the water accessible porosity. Similar effects were reported in the literature for mortars including vegetal fibers [START_REF] Chen | Bio-based ultra-lightweight concrete applying miscanthus fibers: Acoustic absorption and thermal insulation[END_REF]Huyen et al, 2011;[START_REF] Chikhi | Experimental investigation of new biocomposite with low cost for thermal insulation[END_REF]. In general, the authors reported the same decrease in the dry density of mortars with the addition of vegetable fibers, and explained this phenomenon by the influence of multiple factors: i) the lower density of fibers compared to mineral components of the mortars; ii) a modification of the overall granular distribution and internal packing within the bio-based mortars; iii) the contribution of the inner porosity of vegetable fibers and the porosity at the fiber/cement paste interface to the overall porosity of the bio-based mortars, and hence to the drop of dry density. In the present case, fibers are pre-saturated by water, which may also contribute to the overall porosity of the mortar after evaporation of the excess water.

C. Mechanical properties Figure 4 shows the evolutions of the flexural and compressive strengths of the bio-based mortars at 28 days as a function of the weight content of miscanthus fibers.

These experimental data reveal significant decreases in both the flexural and compressive strengths of mortars with the addition of miscanthus fibers. The compressive strength seems most affected among these two properties, as an addition of 2.28 wt.% miscanthus in the mortar induces a strength reduction of 70 %, from 72 MPa to 21 MPa. With higher fiber contents, the compressive strength becomes lower than the minimum requirement for structural concrete (i.e. 17.2 MPa), and hence the corresponding mortars are not suitable for structural applications. Most studies from the literature report similar detrimental effects of the addition of vegetal fibers on the mechanical properties of bio-based mortars. For instance, Chikhi et al. found a decrease in compressive strength of 15% at 28 days for mortar including 10 wt.% date palm fibers of 3mm compared to the neat materials [START_REF] Chikhi | Experimental investigation of new biocomposite with low cost for thermal insulation[END_REF]. Hernandes-Oliviares et al. [START_REF] Hernández-Olivares | Development of corkgypsum composites for building applications[END_REF] and Li et al. [START_REF] Li | Properties of hemp fibre reinforced concrete composites[END_REF]) also observed a decrease in compressive strength with the addition of 20 wt.% of cork granules and various ratios/proportions of hemp, respectively. Such a degradation of the mechanical performance of bio-based mortars can be explained by several factors, in agreement with the conclusions of other authors: i) the bulk density of mortar dramatically decreases, as many pores, voids and entrapped air are introduced with the addition of fibers; ii) fibers may behave like voids under mechanical loading of mortars due to their super-lightweight property; iii) bond defects between the fibers and the cementitious matrix may act as weak points, hence contributing to crack initiation/propagation and to the overall strength loss of the bio-based mortars.

D. Thermophysical properties

Figure 5 presents the evolution of the thermal conductivity (λ) of the bio-based mortars at 25°C as a function of the fiber content. It is found that λ decreases almost linearly with the addition of miscanthus fibers into the mortar (the regression line enclosed in the graph shows a R 2 coefficient higher than 0.99). The same observation was made by several authors for various types of mortars including vegetal fibers [START_REF] Ashour | The influence of natural reinforcement fibres on insulation values of earth plaster for straw bale buildings[END_REF][START_REF] Chikhi | Experimental investigation of new biocomposite with low cost for thermal insulation[END_REF][START_REF] Panesar | The mechanical, transport and thermal properties of mortar and concrete containing waste cork[END_REF]. It is worth noting that the introduction of 5.7 wt. % of miscanthus fibers leads to a decrease in thermal conductivity of the mortar of about 82%, which makes this material highly suitable for insulation applications. This large impact of the fibers on the thermal conductivity can be attributed both to the low conductivity of the miscanthus fibers themselves (a value of 0.04 W.m - E. Moisture Buffer Value The moisture buffer values (MBVs) determined for the various bio-based mortars according to the Nordtest protocol are reported in FIGURE . The MBV clearly increases with the fiber content, as it changes from a value of 0.5 g.m -2 .%RH -1 for the plain mortar to a value close to 2.0 g.m -2 .%RH -1 for mortars with fiber contents over 4.1 wt.%, which corresponds to materials with good or even excellent moisture buffer capacity according to the Nordtest classification.

These results are in accordance with most studies of literature dedicated to bio-based mortars filled with vegetal fibers [START_REF] Kreiger | Moisture buffering in buildings: A review of experimental and numerical methods[END_REF]. For instance, Benmahiddine et al. found an MBV of 2.27 g.m -2 .%RH -1 for hemp concrete [START_REF] Benmahiddine | Experimental investigation on the influence of immersion/drying cycles on the hygrothermal and mechanical properties of hemp concrete[END_REF]. The outstanding moisture buffering capacity of these modified mortars can be attributed to the high intrinsic porosity and specific surface area of the added vegetal fibers [START_REF] Haba | Hygric properties and thermal conductivity of a new insulation material for building based on date palm concrete[END_REF]. In the present study, a good hygroscopic behavior is observed even at low fiber contents, probably due to the use of a micronized miscanthus powder which increases further the specific surface area of fibers.

IV. CONCLUSION

In this work, we explored the possibility of adding micronized miscanthus fibers into a cement mortar formulation, and investigated the effect of this incorporation on the mechanical, thermal and hygric properties of the resulting bio-based mixes.

As a first result, fiber addition was found to reduce significantly the workability of mortars despite the pre-saturation of fibers by water. This effect was mainly attributed to an alteration of the overall granular distribution and packing density of the mortar mixes.

The microstructure of mortars was also highly affected, with a quasi-linear increase in the overall material porosity as a function of the fiber content. This was considered as a main cause responsible for the degradation of the mechanical properties of the bio-based mortars under flexural and compression loading. Contrarily, a great improvement of both the thermal resistance and the moisture buffer capacity was observed after incorporation of small amounts of vegetal fibers into the mortars. The fact that fibers are micronized probably favors this beneficial effect, as their specific surface area is increased compared to fibers of larger size. Finally, the results of the various characterizations lead to the following conclusions regarding the possible usages of the bio-based mortar mixes:

 Mortar M2.5 with 2.28 wt. % of fibers is eligible for structural applications, as it still exhibits a compressive strength over the minimum requirement; but the thermal gain and the capacity of moisture regulation remain limited.

 Mortars with dosages of miscanthus fibers over 4.15 wt.% show substantial improvement of the thermal resistance (over 70%) compared to the plain mortar, and

FIGURE 1 :

 1 FIGURE 1: SEM pictures of the micronized miscanthus fibers used in this study TABLE 1: Composition of the different mortars considered in this study

FIGURE 2 :FIGURE 3 :

 23 FIGURE 2: Slump of the fresh mortar mixes versus the weight content of miscanthus fibers B. Dry density and water accessible porosity Figure 3 shows the evolutions of the apparent dry density and the water-accessible porosity of the hardened mortars as a function of the weight content of miscanthus fibers.

FIGURE 4 :

 4 FIGURE 4: Evolutions of the flexural strength and compressive strength of mortars at 28 days as a function of the weight content of miscanthus fibers.

FIGURE 5 :

 5 FIGURE 5: Evolutions of the thermal conductivity of mortars at 25 °C as a function of the weight fraction content of miscanthus fibers

FIGURE 6 :

 6 FIGURE 6: Moisture Buffer Value of various mortars including miscanthus fibers and comparison with the Nordtest classification.

Total water Water/ (CEM I + CSA) cement ratio

									Mass fraction
									of fibers
									(wt. %)
	M0	1350	1059.33	79.73	2.96	-	398.67	0.35	0
	M2.5	1350	1059.33	79.73	2.96	72.27	604.95	0.53	2.28
	M5	1350	1059.33	79.73	2.96 144.53	843.61	0.74	4.15
	M7.5	1350	1059.33	79.73	2.96 216.80 1092.99	0.96	5.70
	M10	1350	1059.33	79.73	2.96 289.07 1383.85	1.21	6.94

.K -1 is reported in the literature by(Schnabel et al.,

2019)), but most certainly to the large increase in the overall porosity of the mortars resulting from the introduction of fibers, as previously highlighted. Indeed, several authors have suggested that the heat transfer process by conduction may be partially replaced by natural convection in the porous phase[START_REF] Chikhi | Experimental investigation of new biocomposite with low cost for thermal insulation[END_REF][START_REF] Li | Properties study of cotton stalk fiber/gypsum composite[END_REF][START_REF] Li | Properties of hemp fibre reinforced concrete composites[END_REF][START_REF] Onuaguluchi | Plant-based natural fibre reinforced cement composites: A review[END_REF][START_REF] Panesar | The mechanical, transport and thermal properties of mortar and concrete containing waste cork[END_REF].

are classified as good or excellent material for moisture regulation according to the Nordtest protocol. These mortars still retain sufficient mechanical properties for handling and are suitable for insulation applications.

The printability of these modified mortars will be explored in a next study.


I Derbel

M Khemakhem
email: khemakhem.mouna@yahoo.com

S Cerneaux
email: sophie.cerneaux@umontpellier.fr

M Cretin
email: marc.cretin@umontpellier.fr

R Ben Amar

Grafting of low cost ultrafiltration ceramic membrane by Tunisian olive oil molecules and application to air gap membrane distillation

Keywords: Ultrafiltration membrane, Grafting, Olive oil, Membrane distillation

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Grafting of low cost ultrafiltration ceramic membrane by Tunisian olive oil molecules and application to air gap membrane distillation

Imen Derbel, Mouna Khemakhem, Sophie Annick Cerneaux, Marc Cretin,

Raja Ben Amar

To cite this version:

Imen Derbel, Mouna Khemakhem, Sophie Annick Cerneaux, Marc Cretin, Raja Ben Amar. Grafting of low cost ultrafiltration ceramic membrane by Tunisian olive oil molecules and application to air gap membrane distillation. Desalination andWater Treatment, 2017, 82, pp.20-25. 10.5004/dwt.2017.20960. hal-03881898