Gwenaël Bonfante

Mathieu Maillard

Stéphanie Chevalliot

Benjamin Burger

Bruno Berge

Corine Gablin

Failure mode analysis on thermally aged hydrophobic coatings applied to electro wetting

The aim of this study is to investigate the degradation of hydrophobic coatings at the origin of reliability failure. Various properties such as adhesion and wetting after thermal ageing have been investigated. The layers studied, parylene C, Cytop and Fluoropel deposited on silicon wafers are widely used in electrowetting, MEMS and Labon-chip devices. A detailed failure mode analysis is presented, involving chemical surface modification through secondary ion mass spectroscopy by time of flight, X-ray photoelectron spectroscopy, surface energy and tensile strength measurements. We've been able to identify and quantify the origin of failure for each type of coating and the method we applied can be extrapolated to other hydrophobic coating as a benchmark for reliability improvement.

Introduction

Adhesion and durability of coatings on substrates is a major issue in many fields such as painting, adhesive, or wear protection. Understanding the way coatings bond with surfaces remains the more straightforward manner to study rupture mechanisms and long term reliability. For this reason durability of polymer adhesive on different substrates such as ceramic or metals has been studied [START_REF] Venables | Adhesion and durability of metal-polymer bonds[END_REF][START_REF] Comrie | Study of ageing of adhesive bonds with various surface treatments: Part VI. Dicyandiamide-cured epoxy joints aged at 70 °C in a water bath[END_REF][START_REF] Korenberg | Crack growth of structural adhesive joints in humid environments[END_REF][START_REF] Gadre | Contact angle measurements for adhesion energy evaluation of silver and copper films on parylene-n and SiO2 substrates[END_REF]. Improvement from adhesive bonding combined with proper characterization methods prove to be the way to increase durability of adhesive bond, between polymer and metals for example [START_REF] Venables | Adhesion and durability of metal-polymer bonds[END_REF]. More specifically, Comrie et al. demonstrated the impact of moisture associated with heat treatment on adhesive joint and the evolution of reliability [START_REF] Comrie | Study of ageing of adhesive bonds with various surface treatments: Part VI. Dicyandiamide-cured epoxy joints aged at 70 °C in a water bath[END_REF] using a non-destructive method to study and characterize the evolution of joint in a very humid environment. Other improvement on adhesion of paint on plastics involve the composition [START_REF] Dickie | Paint adhesion, corrosion protection, and interfacial chemistry[END_REF], or the adhesion enhancement (plasma, flame treatment or coupling agents) [START_REF] Cho | Improvement of paint adhesion to a polypropylene bumper by plasma treatment[END_REF][START_REF] Sutherland | Modification of polypropylene surfaces by flame treatment[END_REF][START_REF] Garbassi | Surface effect of flame treatments on polypropylene[END_REF]. Garbassi et al. for example, shows the impact of flame treatment on the top surface chemistry and the depth of oxidation for polypropylene pieces. The treated surface is then studied by XPS, TOF-SIMS and contact angle. After several flame treatment, adhesion is promoted thanks to the creation of hydroxyl group on polypropylene pieces surface; and oxidation is shown by apparition of carboxyl group [START_REF] Garbassi | Surface effect of flame treatments on polypropylene[END_REF].

Electrowetting on dielectric [START_REF] Shamai | Water, electricity, and between… on electrowetting and its applications[END_REF] actuation is now widely used for lab-on-chip [START_REF] Fair | Digital microfluidics: is a true lab-on-a-chip possible? Microfluid[END_REF][START_REF] Luk | A digital microfluidic approach to proteomic sample processing[END_REF][START_REF] Rival | An EWOD-based microfluidic chip for single-cell isolation, mRNA purification and subsequent multiplex qPCR[END_REF], display [START_REF] Shamai | Water, electricity, and between… on electrowetting and its applications[END_REF][START_REF] Heikenfeld | Electrofluidic displays using Young-Laplace transposition of brilliant pigment dispersions[END_REF], or liquid lenses [START_REF] Berge | Variable focal lens controlled by an external voltage: an application of electrowetting[END_REF][START_REF] Hendriks | Electrowetting-based variablefocus lens for miniature systems[END_REF] and is based on modulation of wetting properties of electrolyte droplets on surfaces thanks to an electric polarization. It results from the equilibrium of two forces: electrostatic force, which tend to spread electrolyte droplet and surface tension force which tends to limit the expansion of the liquid interface. Even though electrowetting actuation is now available in actual products, many parts of the phenomena remain unknown and need further research since any non-controlled properties modification could lead to device breakdown or dysfunction.

Ageing/breakdown of layers used in electrowetting for instance has been mostly studied when voltage is applied (DC or AC) [START_REF] Hendriks | Electrowetting-based variablefocus lens for miniature systems[END_REF]. Dhindsa et al., studied the self-healing properties of an oxide layer to prevent and repair breakdown [START_REF] Dhindsa | Electrowetting without electrolysis on self-healing dielectrics[END_REF]. Gupta et al. showed the performance loss of layers by pinning the triple contact line [START_REF] Gupta | Impact of pinning of the triple contact line on electrowetting performance[END_REF]. Optimal thickness used in electrowetting on dielectric (EWOD) correlated with hysteresis is investigated by Chae et al. [START_REF] Chae | Optimum thickness of hydrophobic layer for operating voltage reduction in EWOD systems[END_REF]. In addition to operating conditions, product lifetime is also critical for market acceptance and it is important to evaluate thermal ageing while not in use. Indeed, it is very important to get devices lasting for a very longtime both in use and idle.

In this paper, layers commonly used in electrowetting such as Parylene C, Fluoropel and Cytop are thermally aged in water for seven days at 85 °C, to simulate accelerated ageing while stocking. Strong modifications from such treatment are described through surface modification including chemical modification and de-cohesion of layer to identify failure mode for these coatings and improvement strategies.

Experimental details

Ageing procedure

A hermetic sample holder, with a capacity of 12 cells, permits to age any surfaces in any liquid. In our case, we used de-ionized water as we previously established (internal work within Varioptic) that it was critical for both hydrophobic properties and adhesion of the layer studied. In order to have a reproducible ageing procedure, we also used a standardized treatment [START_REF]2-2 -"Essais d'environnement -Partie 2-2: essais -Essais B : chaleur sèche[END_REF], storing samples for one week at 85 °C in 10% of relative humidity in a controlled conditions Binder oven. This ageing procedure is described in IEC 60068-2-2 norm.

Surface energy

The theory behind these methods (one liquid OWKR and two liquids) are used as described by Maillard et al. [START_REF] Bonfante | Two-liquid wetting properties as a surface polarity probe for hydrophobic coatings[END_REF]. Contact angles have been measured on a Kruss easydrop® goniometer associated to the software Drop Shape Analysis using water [START_REF] Bellon-Fontaine | A comparison of thermodynamic approaches to predict the adhesion of dairy microorganisms to solid substrata[END_REF], decane [START_REF] Jasper | The surface tension of pure liquid compounds[END_REF], octane [START_REF] Jasper | The surface tension of pure liquid compounds[END_REF], diodomethane [START_REF] Busscher | The effect of surface roughening of polymers on measured contact angles of liquids[END_REF], ethylene glycol [START_REF] Jie-Rong | Studies on the surface free energy and surface structure of PTFE film treated with low temperature plasma[END_REF][START_REF] Jańczuk | Surface free-energy components of liquids and low energy solids and contact angles[END_REF], and bromonaphtalene [START_REF] Jie-Rong | Studies on the surface free energy and surface structure of PTFE film treated with low temperature plasma[END_REF] as reference liquids. Different coatings have been used such as Parylene C coatings 19 covered or not with fluorinated coatings of Cytop and Fluoropel. Each surface is tested with at least three different liquids for reproducibility. For each tested liquid, three drops of 3 μL are deposited onto the surface, measuring the advancing contact angle in an ambient medium (air or electrolyte). Since substrates are made of 〈100〉silicon wafer, surface roughness is extremely low (ra < 1 nm) and deposition method for Parylene C and top-coats does not significantly increase roughness [START_REF] Jakabovič | Preparation and properties of thin parylene layers as the gate dielectrics for organic field effect transistors[END_REF][START_REF] Daviau | Fabrication of surface plasmon waveguides and integrated components on Cytop[END_REF].

Adhesion

A modified tension-compression machine has been used to test adhesion of samples. Customized pulling test pieces have been made: the section for testing is of about 2.2cm 2 . Then, the different parts of the system are glued with epoxy-amine (Araldite 90s) on the coating side and cyanoacrylate glue (Loctite Superglue 3) on the substrate. Each side of the samples are spread with the corresponding glue, hold for 30 s and finally let to dry overnight. The testing area is delimited by cutting the coating around the pulling test-piece in order to control the de-adhesion surface. Thanks to a software coupled to the machine, the strength leading to the surface break is given and adhesion can be appreciated with the equation: C= F/S with C the constraint in Pascal, F the force in Newton and S the surface in square meters.

Each coating has been tested at least 5 times to be representative and reproducible. Afterwards, the rupture zone is observed with Scanning Electron Microscopy (SEM) and analyzed by energy-dispersive X-ray spectroscopy (EDS) in order to characterize species remaining on the samples and establish failure mode.

Scanning electron microscopy (SEM)

The SEM used is the FEI Quanta 250 FEG allowing us to take pictures between 1 and 10 kV at around 400× magnitude under high vacuum. Each sample were fixed using carbon scotch on sample holder. Parylene C coatings have been metalized with platinum on a Balzers MED 010. Fluoropel and Cytop coatings were not metalized because of their low thickness (≤25 nm), in order to avoid roughness and surface aspect artifacts.

XPS and TOF-SIMS

Measurements were done using a PHI Quantera SXM instrument (Physical Electronics, Chanhassen, USA) equipped with a 180 hemispherical electron energy analyzer and a monochromatized Al K˛ (1486.6 eV) source operated at 15 kV and 4 mA. The analysis spot had a diameter of 200 μm and the detection angle relative to the substrate surface was 45°. Standard deviations were calculated from measurements performed on two different areas. Data were analyzed using the Multipak software. The depth probed of XPS analysis is between 5 and 10 nm. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements were carried out on a TRIFT III ToF-SIMS instrument from Physical Electronics operated with a pulsed 22 keV Au + ion gun (ion current of 2 nA) rastered over a 300 μm ×300 μm area. An electron gun was operated in pulsed mode at low electron energy for charge compensation. Ion dose was kept below the static conditions limit. Data were analyzed using the WinCadence software. Mass calibration was performed on hydrocarbon and fluorocarbon secondary ions. The coatings were analyzed on< 3 nm of depth and under ultra-vacuum.

Results and discussion

Failure test: EWOD

A simple way to see whether or not a layer is aged, is to perform an electrowetting test. Electrowetting corresponds to the modification of wetting properties, from an electrolyte fluid standing on an electrically polarized dielectric coating and it results from the equilibrium of interfacial energy between the spreading electrolyte on a surface and electrostatic energy condensing ions from electrolyte solution near the contact line. Phenomenon occurs whether medium surround electrolyte is air or any non-conducting fluid such as oil and is well described by Young-Lippmann Eq. ( 1): [START_REF] Venables | Adhesion and durability of metal-polymer bonds[END_REF] where θ is the contact angle from the non-conductive medium at a given voltage, θ0 the natural contact angle, ɛ the permittivity of the material and ɛ0 the vacuum permittivity, e is the dielectric thickness, V is the applied voltage and γLG is the interfacial tension between the drop and the medium (usually around 15 mJ/m2 with the liquids used)

The theory describing such phenomenon is well explained by Berge et al. [START_REF] Berge | Électrocapillarité et mouillage de films isolants par l'eau[END_REF] Weathering leads to surface modifications of the layers (hydrolysis, roughness modification, decohesion), leading to hysteresis of voltage [START_REF] Gao | Contact angle hysteresis explained[END_REF][START_REF] Öner | Ultrahydrophobic surfaces. effects of topography length scales on wettability[END_REF][START_REF] Maillard | Two liquids wetting and low hysteresis electrowetting on dielectric applications[END_REF]. In optical devices, accuracy and repeatability is essential, and phenomena like hysteresis must be avoided to remain reproducible and reliable over extensive use.

Fig. 1 describes the effect of ageing on three layers: Parylene C, Fluoropel and Cytop new samples, show almost no hysteresis (Δθ < 5°). On the other hand, aged layers, on the right side of Fig. 1, show two modifications of the electro-wetting behavior. The most noticeable is the increase from starting angle demonstrating a modification of the film surface. Then, hysteresis is very clear on each sample: for a same voltage, raising and dropping angles induce variable contact angle. More precisely for Fluoropel, the behavior's evolution shows a slighter hysteresis. Of course hysteresis here is a major issue for applications, in liquid lens focus strategy for instance, such hysteresis would induce a noticeable focal length difference for the exact same voltage depending on the voltage way of variation. Table 2 presents the hysteresis for each layer studied after ageing. Table 2. Hysteresis after ageing in degree and its correspondence in diopters.

Failure mode analysis

Analysis of the failure mode is based on different characterizations such as coating adhesion, surface morphology, and chemical properties. Indeed, hysteresis is directly linked to modification in the surface properties (morphologically and chemically) induced by ageing the layers. Alteration of such thin films creates anchor points, generating potential barrier for the drop spreading leading to differences on contact angle depending on the path taken. Two hypothesis lead to the formation of these anchor points: loose area or fold generated by decohesion of the underneath layer, creating a pocket in-between layers; direct chemical modification of the top layer, creating polar groups such as hydroxyl groups which are hydrophilic. In order to identify the origin of hysteresis we fully characterize modifications from coatings induced by thermal ageing. The surface energy measurement, using one or two liquids [START_REF] Bonfante | Two-liquid wetting properties as a surface polarity probe for hydrophobic coatings[END_REF], gives a direct estimate of the surface modification by measuring variations in the surface energy polar part. If no change is revealed, adhesion test provide further information when coupled with scanning electron microscopy (SEM) analysis. Finally, in order to fully characterize the top layer (oxidation, hydrophylisation, degradation…), Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and X-ray Photoelectron Spectroscopy characterizations analysis are performed.

Surface energy measurements give a very good appreciation of the coating's deterioration by showing increasing of the polar component. Theory behind the calculation is given by the literature [START_REF] Bonfante | Two-liquid wetting properties as a surface polarity probe for hydrophobic coatings[END_REF].

Polar contribution to surface energy from all tested layers are presented in Fig. 2 and exhibit variations. The Fluoropel layer presents the less variation from 0.004 ± 0.005 mJ/m 2 to 0.01 ± 0.02 mJ/m 2 , however, relative error for both measurements makes them irrelevant. In fact, polar parts for fluoropolymer are too low to be precisely measured and remain unchanged by thermal ageing. The polar part increase on Cytop sample is more noticeable. Indeed, the polar part goes from 0.005 ± 0.001 mJ/m 2 to 0.4 ± 0.02 mJ/m 2 . Finally, the increase for Parylene C is shown on Fig. 2. The polar part goes from 0.1 ± 0.002 mJ/m 2 to 2.5 ± 0.04 mJ/m 2 . Even if, generally speaking, fluoropolymer layers are more resilient to age and weakly interact with their environment, ageing seems to affects samples with some variations. As shown on Fig. 2, all the layers analyzed have initially a low polar part (< 0.2 mJ/m 2 ) but surface polar parts always increase after ageing, showing an alteration of the top layer. Parylene C and Cytop polar part increase readily explain differences observed during the EWOD measurements since ageing could create hydroxyls termination at the coating interface with liquid phase. Now increasing the polar components has a drastic effect on increasing initial contact angle and hysteresis consequently [START_REF] Maillard | Two liquids wetting and low hysteresis electrowetting on dielectric applications[END_REF]. However, Fluoropel hysteresis and starting angle modifications could not be explained from the insignificant variation of the polar part. Two hypothesis arise: de-cohesion of Fluoropel layer from its under-layer (Parylene C) or a top layer's chemical modification not visible through surface energy measurements. Surface energy measurement is usually a good method to probe macroscopically the top surface modifications. However, in the case of Fluoropel, the low polarity criteria proved to be necessary but not sufficient and a deeper analysis is required to understand the ageing process. De-cohesion is investigated thanks to adhesion test coupled with SEM whereas TOF-SIMS, one of the most suitable method to investigate the very top surface, is used to understand the chemical evolution of samples.

Adhesion

The other hypothesis linked to ageing is layer de-cohesion, between or within layers. In order to investigate layer ruptures, adhesion test, especially pull-off test, are performed coupled by SEM and EDS analysis.

Table 3 presents the de-adhesion forces and works for the different layers studied before and after weathering. Film rupture is occurring on/within the top surface, leading to a circular area which can be easily observed by SEM to identify rupture face and breaking mode. As presented in Fig. 3, break occurs, for the layer tested, around 300 N at an elongation of 0.25 mm. The force is then divided by the surface area tested to give a constraint in Pascal (Pa) and a total de-adhesion work of about W= 480 J/m 2 . Fluoropel cohesion is divided by three after ageing coming from 700 J/m 2 to 200 J/m 2 . Cytop adhesion work comes from 650 J/m 2 to 91 J/m 2 after ageing. Parylene is only lightly alterated by ageing, decreasing from 974 J/m 2 to 780 J/m 2 . The wide uncertainty is due to difficulty to normalize the preparation of samples. Aged samples exhibit similar delamination than new samples. EDS analysis exhibits similar peaks, however, chlorine and carbon peaks intensity are drastically reduced: layer is almost completely removed after adhesion test. Ageing slightly weakened the Parylene C interface, as demonstrated by SEM EDS analysis.

Table 3. Experimental data for adhesion testing before and after ageing on

Unfortunately Fluoropel being very thin and sensible to charging effect under electron beam, pictures and image analysis were not straightforward and we could only compare in a qualitative way the differences between new and aged samples after adhesion test.

No real layer's alteration could be noted on Fluoropel: ageing effects on layers' cohesion is visible only by adhesion test. The work needed to pull out is divided by two after weathering. However, the effect is not clearly obvious by SEM clichés (charging effect).

On the other hand, Cytop does not charge under electron beam leading to pictures but no EDS data were gathered because of lack of significant signal. Fig. 5 presents Cytop SEM pictures of a new layer and an aged layer after adhesion test. After ageing and adhesion test, Cytop layer forms patterns. The main hypothesis is that water diffuse through fluorinated layer and condensate or weaken the interface between polymers [START_REF] Sih | Fracture mechanics of adhesive joints[END_REF][START_REF] Banea | Mechanical characterization of flexible adhesives[END_REF][START_REF] Bowditch | The durability of adhesive joints in the presence of water[END_REF]. The patterning could also increase hysteresis by increasing sample's roughness, a very determinant parameter for hysteresis [START_REF] Öner | Ultrahydrophobic surfaces. effects of topography length scales on wettability[END_REF]. No EDS data has been collected because of the poor radiation resistance under the electron beam of this thin film (thickness e= 70 nm).

On the whole samples, ageing in water affects integrity and cohesion of layers, by decreasing adhesion work by a factor of at least two. Parylene C coating adhesion is logically more robust than Fluorinated, taking into account that parylene is much thicker (4.6-5.2 μm) and fluorinated polymer are deposited without coupling agent. The effect of such wearing, could also be shown by SEM coupled to EDS measurements to qualitatively correlate results.

In order to really understand ageing process and appearance of hysteresis, the origin of the degradation has to be identified. To investigate chemical ageing of the top surface, XPS and TOF-SIMS have been performed. Fig. 6 presents negative mode ToF-SIMS spectra in the range of m/ z =15.4-17.2 corresponding to Oe and OHe for a fresh and an aged Fluoropel sample. The Fluoropel is made of a fluoroacrylic polymer of general formula n-C22H14O4F26. Fresh Fluoropel spectrum exhibits only the Oe peak, which is consistent with the presence of oxygen within the polymer. After ageing, the OHe peak (at m/z =17) is detected with a significant relative intensity. Even though it is not straightforward to link a single ToF-SIMS peak to a specific chemical function or group, it is possible in the current case to point that this signature indicates a surface modification consistent with OH terminated chemical functions. The spectrum is dominated by high intensity fluorocarbon signatures (easily detected in ToF-SIMS) but other differences are still identified in terms of hydrocarbon signatures (also more detected). No difference in terms of molecular high mass peaks was detected. From that point of view, it is unclear if the surface modification corresponds to a change in the chemical structure and/or to the detection at the surface of an oxygen containing hydrocarbon contribution. It must be reminded here that surface energy variation was not significant and this could be more consistent with the latter than with the former. According to TOF-SIMS data, each of the layers were modified by ageing in water for one week at 85 °C, even those supposed to be insensitive to moisture and ageing such as fluoropolymers.

XPS analysis was also performed on Fluoropel in order to give a complementary view on the top layer surface chemical modification to those from ToF-SIMS and surface energy measurements.

Table 4 presents the atomic percentages and ratios for the main atoms composing Fluoropel coatings (Cl was also slightly detected but its atomic percentage is not displayed in Table 4). As indicated above, it is based on the repetitive unit of formula n-C22H14O4F26. Oxygen and carbon atomic percentages increase from ~12% to ~17% and ~35% to ~45% respectively, while fluorine atomic percentage decreases from around 50% to ~32%. Fluorine was thus lost during the process of ageing, in favor of carbon and oxygen. One of the main hypothesis for this loss is the hydrolysis leading to the release of a very volatile compound known as 1H, 1H, 2H, 2H-Perfluoro-1-octanol. This is displayed in Fig. 8 in the case of one of the two fluoro-alkyls but both could undergo this reaction. This hypothesis is based on the fact that oxygen in the ester group is the weak point of the polymer backbone, possibly undergoing reverse esterification in presence of water: RCOOR′+ H2O↔ RCOOH + R′OH. XPS being based on atomic percentages, comparison of these percentages to the theoretical proposition from Fig. 8 may lead to the elucidation of the degradation mechanism. Before ageing, theory calculation values are based on the expected formula n-C22H14O4F26: each recurring unit contains 26 fluorine atoms, 22 carbon atoms and 4 oxygen atoms, for a total of 52 atoms that can be detected by XPS (thus excluding hydrogen atoms, not measured by XPS). It must be noted that the measured atomic percentages, even if falling in the expected range (especially for Fluorine), exhibit some differences (especially for O) when compared to the Fluoropel composition (Chlorine traces is also an indication that surface is not in pristine condition). 4) after ageing makes possible to study the extent of side alcohol removed from the polymer per recurring unit. Considering x as the number of side alcohol released by hydrolysis per recurring unit (x being comprised between 0 and 2), the fluorine content is given by: = When considering loss of fluoro-alcohols, the total amount of atoms than remain unchanged is only given by the 4 oxygen atoms and the 6 carbon atoms behind the ester group. Therefore, 13 fluorine atoms and 8 carbon atoms per fluoro-alkyl part could be removed with the alcohol, they are thus included as (2-x) as maximum two alcohols released (xmax = 2). The expected atomic content for carbon and oxygen are similarly calculated and in qualitative agreement with experimental data (see Table 4).

Hence the number of side alcohols removed from the polymer (1.49 entity) is higher than 1, i.e. at least one fluoro-alcohol is released per recurring unit. Hydrolysis of the ester group leaves a carboxylic acid function at the very top surface. However, about half the flexible fluoroalkyl chains remain on one side, potentially covering the potential carboxylic acid groups: no changes are visible using surface energy measurements and a spectrum dominated by fluorocarbon peaks of the ToF-SIMS analysis (even more top-surface sensitive than XPS). For the latter, OH terminated chemical groups were still detected, giving a first indication of this surface modification. These results correlate with TOF-SIMS measurements in which OH terminated chemical groups were detected.

Hydrolysis reaction hypotheses can also be made for the two other layers: -Cytop undergoes ring opening. (Fluorine create tension in the molecule by strongly polarizing the CeF bond), weakening the carbon oxygen bond by electron's delocalization) (Fig. 9).

-Parylene C chlorine is hydrolyzed, and, forming an hydroxyl group [START_REF] Bunnett | Aromatic nucleophilic substitution reactions[END_REF] on the aromatic ring (Fig. 10).

Conclusion

Characterizing layers used in electro-wetting is the key to obtain reliable devices. Each coating reacts in a different way to ageing, even though final result is somehow similar, with apparition of a wetting hysteresis. Parylene C when aged in water at 85 °C, exhibits the largest modifications in term of surface energy and thus in term of wetting whereas layer cohesion is only slightly modified by weathering. Cytop surface energy is less modified by ageing than parylene C. However, top surface analysis demonstrated that the coating is chemically modified, creating anchor point and thus hysteresis. Finally, there is no significant modification of Fluoropel surface energy but TOF-SIMS analysis still exhibits hydroxyl group on the top surface whereas XPS points out hydrolysis of a lateral chain. No noticeable modification is observed on surface energy polar part: since lateral chains are mobile, hydroxyl groups can be eventually covered and layer behaves as if non-aged. Chemical ageing is not easily overpassed. It needs modifying directly the chemical structure of the coating, for example using the method presented by Glaris et al. [START_REF] Glaris | N2 plasma-assisted grafting of fluorinated chains onto partially cured epoxy resins[END_REF], grafting fluorinated chain directly on the coating. Physical ageing (such as de-cohesion, dwelling …) can be easily improved by adding charge or anti-oxidant products directly to the coating solution. Adhesion can be promoted by adhesion agent, as shown by Hegemann et al. [START_REF] Hegemann | Plasma treatment of polymers for surface and adhesion improvement[END_REF], or by depositing an anchor coating, shown by Papathanasiou et al. [START_REF] Papageorgiou | Superior performance of multilayered fluoropolymer films in low voltage electrowetting[END_REF].

These results demonstrate that wetting hysteresis from hydrophobic coating used in electrowetting applications originate from various parameters, and a relevant failure analysis requires in some case advanced analytical measurements such as TOF-SIMS and XPS. Now, tests performed on materials must be easy, reproducible and quantitative in order to make them applicable to industrial standard. Based on our analysis, tests could be divided in two types: routine and advanced reliability tests. Each coating is required to go through routine reliability test (such as here, electrowetting, surface energy or adhesion) before considering more advanced analysis. Our results demonstrate a cost effective failure analysis methodology, applicable to coatings industrial batches as well as newly designed hydrophobic coatings used electrowetting based actuator and applications involving hydrophobic coatings.

2. 1 .

 1 Sample preparation Parylene C coatings have been prepared on silica substrate by CVD using a Kisco system. The Dichloro-di-para-xylylene cyclic dimer precursor has been provided by Kisco as diX C. Cytop® CTX 809 has been supplied by AGC Chemicals. Hydrophobic coatings based on fluorinated polymers have been obtained by spin coating of a solution of Cytop® directly spun on the latter dielectric coatings of Parylene C at 4000 rpm for 90 s, Fluoropel® 1601 V has been supplied by Cytonix Co. The asreceived product has been spun at 6000 rpm on dielectric coatings of Parylene C. Samples are then annealed at 90 °C for> 15 h on a hot plate. Three types of samples are studied: pristine Parylene C substrate, a Fluoropel covered Parylene C and Cytop covered Parylene C. Parylene C coating is 5.6 μm thick and additional layer of Cytop and Fluoropel are about 50 nm thick (see

Figure 1 -

 1 Figure 1 -EWOD for Parylene C (square), Fluoropel (circle) and Cytop (triangle) showing the contact angle as a function of the voltage (1kHz frequence, in liquids specially designed by Varioptic) before (left) and after (right) thermal ageing.

Figure 2 -

 2 Figure 2 -Polar part modification measured with the two liquids method before and after ageing in water for seven days at 85°C in 10% humidity for Parylene C (triangle), Cytop (circle) and Fluoropel (square).

  Fluoropel, Cytop and Parylene C.

Figure 3 -

 3 Figure 3 -Typical Curve obtained from an adhesion test on a layer. Breaking point (arrow) is observed around 320 newton, correspond to de-adhesion work of roughly 260J/m².

Fig. 4

 4 shows two pictures of pulled off Parylene C layers new (left) and aged (right) taken by SEM. For both samples, there is delamination of the top layer leading, to a complete de-adhesion of Parylene C from the silicon wafer. To better appreciate the quantity of layer removed, EDS has been performed on dark grey area (remaining part of the top layer) and light grey area (partial or complete removal). The left EDS spectrums correspond to the non-aged surface of Parylene C. The top left one is linked to dark area, bottom left is the spectrum for light grey area. Chlorine and carbon peaks are linked to chlorinated polymer Parylene C (C8H7Cl) and silicon peak corresponds to the wafer underneath. Before pull-off test, only Parylene C is visible by the EDS detector. Removed area shows silicon peak with Parylene C's peaks: a part of the layer is pulled decreasing the thickness of the remaining layer.

Figure 4 -

 4 Figure 4 -SEM pictures of new (left) and aged (right) Parylene C samples and coupled EDS analysis on remaining layer (top) and pulled off (down).

Figure 5 -

 5 Figure 5 -SEM pictures of new (left) and aged (right) Cytop samples.

Figure 6 - 2 Fig. 7

 627 Figure 6 -Negative mode ToF-SIMS spectra of Fluoropel new (left) and aged (right) in the range of m/z = 15.4-17.2 Fig. 7 describes negative mode ToF-SIMS spectra of Cytop layers before and after ageing in water. As described for Fluoropel, a peak appears around 17 m/z on the weathered layer, exhibiting a possibly similar change at the top surface.

Figure 7 -

 7 Figure 7 -Negative mode ToF-SIMS spectra of Cytop new (left) and aged (right) in the range of m/z = 15.4-17.2

Figure 8 -Figure 9 -

 89 Figure 8 -Hydrolysis of Fluoropel leading to the release of a fluoro-alcohol

Figure 10 -

 10 Figure 10 -Hydrolysis of Parylene C

Table

Table 1 .

 1 Thickness and capacitance of layers studied in electrowetting.

Table 4 .

 4 XPS atomic percentages of C, O and F for Fluoropel samples before and after ageing on two different locations on samples (point 1 and point 2) and comparison to theory calculation (see text for theory calculation after ageing).

Acknowledgement

We would like to thank Science & Surface from Lyon for the providing of XPS analysis on our samples, their quick answer and their expertise. This work was supported by the Association Nationale Recherche Technologie within the program "Conventions Industrielles de Formation par la Recherche" N° 2014/0506 and by the business unit Varioptic.


G Bonfante Ab

S Chevalliot

B Toury

B Berge

M Maillard
email: mathieu.maillard@univ-lyon1.fr

Two liquids wetting properties as a surface polarity probe for hydrophobic coatings

A model describing a non-polar liquid spreading on a surface within a polar medium is described theoretically, according to Fowkes Good and Girifalco approximations on interfacial tension. We demonstrate both theoretically and experimentally that surface polarity measurements using contact angle from two immiscible liquids minimize drastically the measurement error. The present method has been successfully applied to various substrates of variable polarity and overall surface energy. We also demonstrate that this method allows a direct measurement of surface sensitivity to pH.

Introduction

Surface energy and more specifically introduction of polar and apolar components by Fowkes 1,2 in the 50's have been a breakthrough in the understanding of not only liquid wettability but also more complex systems implying a competition of spreading or adhesion of two or more compounds 3 on a surface due to a hydrophilic -lipophilic balance between compounds. A fairly large scientific and technologic domain concerning non adhesive coatings relies on reliability of non-polar coatings like Micro Electro Mechanical Systems (MEMS) or optical coatings. Adhesion of proteins and living cells 4 in organisms are also related to adhesion in a surrounding medium 5-7 . This has been made possible by an extended theoretical work from the Young's equation 8 in the early 19 th century to more recently the establishment of the method 9,10 from Owens, Wendt, Rabel and Kaelble (OWRK), measuring contact angle from at least two liquids of known surface tension, and having a large chemical difference in terms of polarity. This method is, now, widely used and very well adapted for most surfaces. It is based on the contact angle measurement of one liquid surrounded by its vapor, but consequently it does not mimic exactly the situation where adhesion or spreading from one element occurs in a condensed medium. In this case, spreading is a competition between two compounds chemical affinity with a surface. Also, measuring contact angle from a non-polar liquid in absence of water, does not take into account specific interactions of aqueous or polar medium with the substrate of interest. In particular, it cannot take into account that surface somehow reacts with one of the liquid phase, by pH-dependent proton exchange, surfactant adsorption or material swelling for instance 11 . Some alternative method 2 using contact angle hysteresis 12 and electron donicity 13 have been developed, in particular for biological interactions, but are also limited to low surface energy since they imply only one liquid spreading at a time, on a surface.

Theory

The most standard method for determination of surface energy and surface polarity is based on Owens, Wendt, Rabel and Kaelble (OWRK) method 9,10

. This method relies on Young's equation combined with Owen, Good and Girifalco approximation. Young's equation describes contact angle from a drop of liquid at equilibrium of interfacial forces at the triple line between a supporting solid, the liquid and its vapor:

(1) with the surface energy, solid-liquid interface tension and liquid-vapor surface tension. Owen, Good and Girifalco approximation is based on dispersive and polar interactions, and describing interfacial tension as a function of individual surface tensions:

(2) With and corresponding to polar and dispersive components from liquid surface tension and and corresponding to polar and dispersive components from surface energy. Combining equations 1 and 2 leads to a linearized equation as a function of both polar and dispersive components from surface energies:

(3) leading to a linear equation Y=AX+B with and .

In this equation the square root of polar and dispersive surface energy components correspond to the slope and y-intercept of the affine function, respectively.

Increasing the number of measurements, further improves accuracy of polarity measurements but substrates can be sensitive to probing liquids. In addition, it is sometimes required to use a fresh sample for each measurement because a previous liquid deposition can influence the next measurement, which makes it more complex in terms of sample reproducibility. Increasing the required number of measurements obviously makes it also more difficult for a routine test in industrial applications. This method is also limited to low surface energies, since liquids having a lower surface tension than supporting substrates are in a total spreading situation according to Zisman law 14 , which of course prevents from any surface polarity measurement on high surface energy substrates.

Experiments

One and two liquids contact angle measurements have been performed on a Kruss easydrop® goniometer and contact angle has been measured with the associated software Drop Shape Analysis using water , and bromonaphtalene 18 as reference liquids. Aqueous solutions of variable pH have been obtained diluting hydrochloric acid and sodium hydroxide in ultra-pure water. In the present work, we used Parylene C coatings 20 and SiOC, covered or not with fluorinated coatings of Cytop and fluoropel as substrates (see details in ESI S1).

Each surface is tested with at least four different liquids. For each tested liquid, three drops of 3µL are deposited onto the surface, measuring the advancing contact angle in an ambient medium. Since substrates are made of <100> silicon wafer, surface roughness is extremely low (r a <1nm) and deposition methods for Parylene C and top-coats do not significantly increase roughness 21,22 .

Results and discussion

Based on Fowkes, Good and Girifalco approximations and previously published results [START_REF] Zisman | Contact Angle, Wettability, and Adhesion[END_REF] we develop a predictive model for surface polarity based on two liquids wetting: assuming an oil drop medium (o) surrounded by a polar and immiscible medium (w) standing on a flat surface (s) we can write an extended version of Young's equation:

In the same way, interfacial tension expressions of  sw and  so are injected into the extended Young's equation:

The liquid-liquid interfacial tension ow can be either calculated from the same approximation or measured experimentally. Key point from this model is in equation 4: on first approximation, contact angle is sensitive to surface component difference between oil and polar phases either dispersive or polar. As opposed to the corresponding equation from OWKR's method (eq. 2), this term can eventually be neutralized for liquids having similar dispersive component whereas polarity is intrinsically very different between a polar and a non-polar liquid. According to this, contact angle in two liquids has most of the time a stronger dependency on surface polarity than dispersive component. This effect explains qualitatively why two liquids measurement is intrinsically more sensitive to surface polarity when using a polar and a non-polar liquid.

Now similarly to OWKR method, equation 4 leads to a new linearized equation:

(

with and

Dispersive and polar components are now given by the square of slope and y-intercept respectively: and The consequence is that polar component  s p now relates to the y-intercept from this equation whereas in the one-liquid method it relates to the slope of the affine function.

The reason for this inversion relates to uncertainty: using a reduced coordinate Y divided by a value close or equal to zero, makes the error diverges, so we deliberately choose to divide by polar components difference. Conversely, in the one liquid method, the dispersive component is the divider, for the exact same reason.

In order to compare latter methods, we used various polar and non-polar reference liquids (table S2 in ESI) to measure contact angles on various low polarity surfaces including Parylene C, a high surface energy coating of  s =35mN/m (figure 1). We used equation 5 to determine polar and dispersive components from coatings and compared to OWKR method using same coatings or batches of coatings, but measuring one liquid at a time in air. Interfacial tensions between liquids used in the two liquids method are calculated according to equation 2 and given in ESI table S3.

Figure 1: Linearized coordinates X and Y according to equations 3 and 5 measured experimentally (triangles) on Parylene C according to OWKR method (left) and 2-liquids method (right) with liquids indicated in table S2 and interface tension indicated in S3. On both graphs, full line is the linear regression and dashed line is the confidence interval of 95% according to Student's law.

OWKR's method is very accurate for moderate or relatively high polar surfaces but has an intrinsic limitation for low polarity coatings where a large error is observed in the determination of slope and polar component consequently.

Based on figure 1, one obvious observation is that we obtain a much better linear estimation of y-intercept and slope with a coefficient of regression r²=0.97 with 2-liquids compared to r²=0.61 with the OWKR method.

Estimated standard deviation from slope A and y-intercept B can be made from a statistical analysis Based on this calculation one obtains the results described in figure 1 and table 1. As can be noted, polar components are much more accurate using the 2-liquids method than the OWKR method. In order to understand the origin of this difference, we also calculated intrinsic error from polar and dispersive component from partial derivative on both one and two liquids models for substrate of variable polar and dispersive component: Main difference between the 2 methods is illustrated on figure 2: the 2-liquids method is more sensitive to polar component than dispersive interaction, and as a consequence the method exhibits a much larger intrinsic error on dispersive component measurement (Fig. 2A). Inversely, relative error on polar component is intrinsically lower and does not diverge as the OWKR method when polar component tends to zero (Fig. 2B). This particular feature is linked to the fact that, when polar component tends to zero, with one liquid in air, it will necessarily lower contact angle , as overall surface energy increases, whereas using two liquids, contact angle from non-polar liquid will tend to zero. In order to validate this new method, we voluntarily performed the same measurements but limiting to three measurements, i.e. water, ethylene glycol and diodomethane for the OWKR method and dodecane, bromonaphtalene and diodomethane in water for the 2 liquids method. This experiment is performed on two low energy fluorinated coatings, Fluoropel and Cytop, and SiOC oxycarbide. It remains obvious from figure 3 that measurement with 2 liquids provides a much more precise measurement of polar component according to regression coefficients and estimated error. When standard method using only three drops of different reference liquids is not accurate enough to estimate polarity from a given surface, two liquids method proves to be largely efficient enough for this kind of determination. The other huge advantage of this method is to be able to measure surface polarity as a function of pH in an aqueous environment. In this case contact angle from non-polar liquids is measured in water of variable pH. This of course is not possible when measuring only one liquid, since only water of variable pH will induce a modification of contact angle. In addition, as expressed by equation 4, contact angle variation is much more sensitive to polar component in 2-liquids method than in OWKR's. We performed this experiment on Fluoropel and Parylene C and observed from figure 4 that Parylene C exhibited a much larger polarity variation with pH. This result is consistent with literature 25,26 describing parylene as a coating with a negative zeta potential of -17.5mV due to the presence of carboxylic groups at the surface of Parylene, related to superficial surface oxidation under oxygen and/or UV light. Our results are also supported by chemical considerations on structure of Parylene C and fluoropel (ESI S5), as Parylene is a halogenated polymer and potentially exhibit surface carboxylic acid group whereas fluoropel only has ester function, that are very weakly hydrolysable, leading to both non polar properties and a better chemical inertia toward pH variation. Since zeta potential measurement on flat surfaces usually involve electroosmotic current measurement 27 that are experimentally complex, this method could eventually lead to an easier alternate technic.

This measurement capability can also be used to probe differential affinity of a lipophilic compound in a hydrophilic environment, like proteins or organic compound dispersed in water, as wetting and surface tension are directly linked to adhesion.

Conclusions

We developed a measurement method based on a two liquids contact angle measurement, allowing much more precise determination of surface polarity, especially on low surface polarity surfaces. This method is also more accurate when performing measurements with a limited number of assays. More importantly, it also allows probing surface properties as a function of pH, measuring surface energy for complex systems where wetting or adhesion is a competition between two compounds varying in lipophilic-hydrophilic nature.

Notes and references

  and the estimated values of calculated from measured values slope and y-intercept and . Estimated values from A and B are then expressed with an error of with the Student's tdistribution law providing uncertainty according to a trust percentage .

  in the one liquid case we can deduce error from equation 3:and relative error on surface polar component and dispersive componentWhereas errors from equation 4 are given by: and Cross calculation on relative error on polar component is not displayed as a function of dispersive component, as it remains constant, similarly to dispersive component error a function of polar component.

Figure 2 :

 2 Figure 2: Relative error on polar component calculated on a low surface energy coating  s =20mN/m of variable polarity (A) and relative error on dispersive component on a moderately polar surface (0.5mN/m) of variable dispersive component (B). Red line is the calculated error related to the classical OWKR method with water as a reference liquid whereas blue line is the two liquids method with water and dodecane as reference liquids. Contact angle error was estimated from experimental values as =0.4rad

Figure 3 :

 3 Figure 3: Linearized coordinates measured experimentally (square) on Cytop (1), Fluoropel (2), and oxycarbide SiOC (3) according to the OWKR method (A) and 2-liquids method (B). On both graphs, full line is the linear regression and dashed line is the confidence interval of95% according to Student's law. Results have been obtained with only three measurements.

Figure 4 :

 4 Figure 4: Polar surface energy components on Parylène C (squares) and Fluoropel (triangles) as a function of water pH (measured with diiodomethane, Bromonaphthalene and octane).

Table 1 :

 1 Dispersive (

			1 liquid		2 liquids
	Table 1	ɣ (mJ/m²)	Ɣ d (mJ/m²)	Ɣ p (mJ/m²)	Ɣ p (mJ/m²)
	Parylene C	40.8 ± 0.3	40,1 ± 0.15	0.7 ± 0.013	0.1 ± 0.001
	Fluoropel	11.30 ± 0,05	11.1 ± 0.02	0.3 ± 0.03	0.01 ± 0.005
	Cytop	13.3 ± 0.07	13.0 ± 0.03	0.05 ± 0.03	0.01 ± 0.003
	SiOC	23.6 ± 0.05	23.2 ± 0.03	0.06 ± 0.06	0.08 ± 0.05

d ) and polar ( p ) surface energy component measured from 1 and 2 liquids methods on various substrate (ESI S4

).