[
    {
        "id": 1,
        "chunk": "# Surface wettability and stability of chemically modified silicon, glass and polymeric surfaces via room temperature chemical vapor deposition  \n\nVania Silverioa,b,⁎, Patricia A.G. Cananea, Susana Cardosoa,b  \n\na INESC Microsystems and Nanotechnologies, INESC MN, 1000-029 Lisboa, Portugal b Department of Physics, Instituto Superior Tecnico, Universidade de Lisboa, 1040-001 Lisboa, Portugal",
        "category": " Abstract"
    },
    {
        "id": 2,
        "chunk": "# G R A P H I C A L A B S T R A C T  \n\nRoom temperature chemical vapor deposition was used to modify surface wettability of materials commonly used in the fabrication of microfluidic devices: rigid flat surfaces of silicon (Si), glass, SU-8 photoresist and PDMS (polydimethylsiloxane). The efficiency of surface coverage and consequently the efficiency and stability of surface wettability modification are seen to be highly dependent on the availability of –SiOH groups at the surface and the time of surface activation. Surface wettability after CVD modification was perceived to be governed by the wettable nature of the tail group deposited at the surface.  \n\n![](images/caf47779fc10d47d9d80f32c20fe284d6dd08c7349fd90967cc4aa0b66ef9a62.jpg)",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# A R T I C L E I N F O",
        "category": " Abstract"
    },
    {
        "id": 4,
        "chunk": "# A B S T R A C T  \n\nKeywords:   \nSurface wettability modification   \nRoom temperature chemical vapor deposition   \nMicrofluidics   \nMicrofabrication  \n\nWettability of surfaces used in microdevices, either on the fabrication of fluidic passages or integration of sensing and actuating elements can impact the flow. The easiness of fabrication may dictate the materials and processes to use, that can subsequently have their surface wettability spatially controlled. The work reports surface wettability modification (SWM) by room temperature chemical vapor deposition (CVD) with HMDS (hexamethyldisilazane) and FDTS (perfluorodecyltrichlorosilane) on flat surfaces of silicon (Si), glass, SU-8 photoresist and PDMS (polydimethylsiloxane). The effect of SWM has been evaluated by the measurement of contact angles (CA) of $6{\\upmu\\mathrm{L}}$ droplets of deionized water and phosphate-buffered saline buffer (PBS). Time of surface exposure and evolution of CA after modification have been investigated for each pair surface/fluid. The time of surface activation with HMDS or FDTS and the chemical affinity of these to the surface are seen to govern the efficiency of surface coverage and consequently the efficiency and stability of SWM. For both HMSD and FDTS SWM, hydrophilic surfaces (glass and Si) became more hydrophobic (CA rising from $20^{\\circ}$ up to ${\\bf\\tilde{\\Gamma}}^{70^{\\circ}}$ ) while SU-8 hydrophobic surfaces became more hydrophilic (CA decreasing from $120^{\\circ}$ down to $\\mathrm{\\tilde{\\tau}}_{100}\\mathrm{\\cdot}$ upon $30\\mathrm{min}$ activation. PDMS surfaces shows no relevant SWM after activation with HMDS nor with FDTS. SWM of Si surfaces has remained irreversible following CVD exposure (HMDS and FDTS) for at least $65\\mathrm{h}$ .",
        "category": " Abstract"
    },
    {
        "id": 5,
        "chunk": "# 1. Introduction  \n\nMicrofluidic devices have received much attention in the past years due to their competitive advantages, especially regarding their reduced sample and reagent consumption, analysis time and increased automation. To build and use microfluidic passages, the characteristics of the materials at hand must be considered: minimum/maximum operating temperature, resistance and conductivity, electroosmotic mobility which influences analyte affinity, surface energy that plays an important role in surface wettability, among others. PDMS (polydimethylsiloxane), silicon, glass and SU-8 photoresist are all widely used materials in the fabrication of microfluidic devices [1–3]. Therefore are now references when novel surfaces are addressed for microchannel fabrication.  \n\nMicrofabrication and usage of microfluidic devices are extremely dependent on forces at the surface. Hydrogen bonds and van der Waals forces are sufficient to promote unwanted adhesive joint between PDMS microfluidic structures and SU-8 masters, adding difficulty in device peeling. On the other hand, adhesion forces between polymers (usually materials of high surface energy) and glass or metallic foils (usually low surface energy materials) are very week and require additional chemical reactions of highly reactive functional groups at the interface to promote strong covalent chemical bonds [4]. Surface wettability modification (SWM) appears as a very useful tool to tune these materials surface energy [5–7] not only for adhesion purposes for example in microfabrication or microfluidics [8,9], but also for inhibition of nonspecific adsorption of analytes at the surface [10,11], anti-stiction surfaces [12] or self-cleaning surfaces or even enhanced capillary pumps [13].  \n\nSurface wettability modification techniques, can be achieved by gasphase processing, wet chemical methods or a combination of both (Table 1) [14]. Electrochemical anodization and electrospinning techniques are also used for SWM. Compared to the other SWM processes, described elsewhere [15–20], Chemical Vapor Deposition (CVD) is a fairly simple technique in which the deposition of vaporized molecules form a thin film when in contact with a surface [21,22]. CVD is very attractive also because the vapor molecules can conform to the geometry of the substrate at relatively high deposition rates only requiring rough to process vacuum regimes [23]. The small amounts of chemicals used and the ability of the vapor-phase not to transport impurities present in the liquid to the thin film layer make this technique extremely appealing [24,25].  \n\nSan Vicente and co-workers [26] studied the effect of HMDS (hexamethyldisilazane) on wettability of porous antireflective (AR) coatings for solar glass covers. The surfaces consisting of glass with antireflective coating, rich in residual silanol groups, were immersed in HMDS/ Hexane with varying concentrations $(0\\%$ to $100\\%$ HMDS), for a range of immersion times from 0 to $1400\\mathrm{min}$ , at room temperature. The authors intended to minimize the number of silanol groups present at the glass surface since these are very reactive. Silanol groups induce the adsorption of water vapor and contaminants under humidity conditions, specifically leading to deterioration of optical properties of these AR films. The authors could successfully increase an initial static contact angle (CA) of $25^{\\circ}$ to $105^{\\circ}$ after the treatment with $100\\%$ (pure) HMDS solutions and reaction times longer than $60\\mathrm{min}$ .  \n\nStiction forces can have negative impact on the fabrication and application of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) as they compromise reliability, longterm stability, efficiency and durability of the devices. Zhuang et al studied surface anti-stiction coatings of self-assembled monolayers of several organosilane precursors including FDTS (perfluorodecyltrichlorosilane), grown in vapor phase on silicon surfaces [27]. The antistiction performance was evaluated by the $\\mathbf{CA},$ among others. In this study, the vapor-phase coating process was performed for process pressures down to 0.2 mbar and temperatures between $20^{\\circ}\\mathrm{C}$ and $300^{\\circ}\\mathrm{C}$ . The authors could obtain contact angles of $115^{\\circ}$ for modified silicon wafers, which are naturally hydrophilic. Other techniques have been pursued to control surface adhesion in numerous applications [28–30].  \n\nAs seen above depending on the final application the surface interfacial energy can be tuned to the desired surface wettability and the SWM determined. The wettability of a liquid to a solid surface is characterized by the measurement of contact angle (CA). The relation between contact angles and surface energy is given by the Young equation:  \n\n$$\n\\gamma_{\\mathrm{lv}}\\cos(\\Theta)=\\gamma_{\\mathrm{sv}}-\\gamma_{\\mathrm{sl}}\n$$  \n\nwhere θ is the contact angle, $\\upgamma_{\\mathrm{lv}}$ is the liquid-vapor interfacial energy, $\\upgamma_{\\mathrm{sv}}$ is the interfacial energy between solid surface and vapor and $\\upgamma_{\\mathrm{sl}}$ is the solid-liquid interfacial energy. Contact angles $\\theta\\:<\\:90^{\\circ}$ correspond to high surface energy or high wettability (hydrophylic, fluidofilic or lyophilic surface), while contact angles $\\theta>90^{\\circ}$ correspond to low surface energy or low wettability (hydrophobic, fluidofobic or lyophobic surface) [31].  \n\nThe need to use various materials in the fabrication of integrated microfluidic devices with sensing and actuating elements, several microfluidic structures, etc. directly impacts the microfabrication effectiveness. The static contact angle provides valuable information of surface wettability useful to estimate microfabrication efficiency to support functional multilayers [32–34]. Further information on the impact of surface chemical heterogeneity or macroscopic roughness can be accessed by the dynamic contact angle [35].  \n\nParticularly, in this work a room temperature gas-phase processing (Chemical Vapor Deposition, CVD) with HMDS and FDTS was used to modify the interfacial energy of different surface materials (PDMS, glass, SU-8 and silicon). The contact angle measured by sessile static drop method with DI water and PBS buffer solution allows the classification of the effects of surface wettability modification. Image J software with LBADSA (low-bond axisymmetric drop shape analysis) plugin, developed by Stalder and co-workers [36] as a tool to determine the CA from droplet images, was used to obtain the CA in this work. The approach is based on a first-order perturbation technique to analytically solve the Young-Laplace equation (Eq. 2) and provide the whole sessile drop contour and as a result, the contact angle.  \n\nTable 1 Examples of gas-phase and wet chemical surface wettability modification (SWM) techniques.   \n\n\n<html><body><table><tr><td>Gas-phase processing</td><td>Wet chemical methods</td></tr><tr><td>ultrasonic spray pyrolysis calcination</td><td>phase inversion</td></tr><tr><td>hydrothermal treatment</td><td>interfacial polymerization</td></tr><tr><td></td><td></td></tr><tr><td>ultraviolet irradiation</td><td>layer-by-layer deposition</td></tr><tr><td>microwave irradiation</td><td>sol-gel coatings</td></tr><tr><td>plasma oxidation</td><td>silanization</td></tr><tr><td>UV irradiation</td><td>dynamic modification with surfactants</td></tr><tr><td>chemical vapor deposition</td><td>protein adsorption</td></tr><tr><td>atomic layer deposition</td><td></td></tr><tr><td>sputter coating of metal compounds</td><td></td></tr></table></body></html>  \n\n$$\n\\Delta\\mathfrak{p}=\\gamma\\ \\nabla\\cdot\\boldsymbol{n}\n$$  \n\nThe Young-Laplace equation relates the interfacial tension $\\gamma$ between the two fluids and the pressure difference across the interface Δp in opposite direction to $n$ . Here n is a unit vector normal to the interface directed from the liquid drop to air.",
        "category": " Introduction"
    },
    {
        "id": 6,
        "chunk": "# 2. Experimental technique",
        "category": " Materials and methods"
    },
    {
        "id": 7,
        "chunk": "# 2.1. Materials  \n\nRigid flat samples of glass microscope slides $(76\\times26\\times1\\mathrm{{mm}}$ thick, Normax), silicon pieces (single side polished Si, mechanical grade, $0.65\\mathrm{mm}$ thick, University Wafer), SU-8 2005 coatings $(5\\upmu\\mathrm{m}$ thickness, permanent epoxy negative photoresist, Microchem) and PDMS membranes (10:1, $0.5\\mathrm{mm}$ thickness, SYLGARD ${\\mathfrak{P}}$ 184 silicone elastomer, Dow Corning) were exposed to HMDS vapor (hexamethyldisilazane $\\mathrm{(HN[Si(CH_{3})_{3}]_{2}}$ , $161.40\\mathrm{g.mol}^{-1}$ , $96.0\\%$ , TCI) and FDTS vapor (perfluorodecyltrichlorosilane, $(\\mathrm{CF}_{3}(\\mathrm{CF}_{2})_{7}(\\mathrm{CH}_{2})_{2}[\\mathrm{SiCl}_{3}],$ $581.56g.\\mathrm{mol}^{-1}$ , $97.0\\%$ , Alfa Aesar) at room temperature. The investigation of SWM consist in measuring the contact angle of liquid drops of deionised (DI) water or water-based phosphate-buffered saline solution (PBS, $1\\times$ , $\\mathsf{p H7.4}$ ) in contact with the solid surface after CVD exposure",
        "category": " Materials and methods"
    },
    {
        "id": 8,
        "chunk": "# 2.2. Surfaces preparation  \n\nsamples of glass and silicon have been washed with Alconox® anionic detergent for $3\\ensuremath{\\mathrm{h}}$ , rinsed with isopropanol $(>99.8\\%$ , Labchem) followed by DI water, and blow dried. PDMS membranes have been prepared by manually mixing 3 dimethyl siloxane and 184 silicone elastomer (cross linking agent) in 10:1 ratio, left for $^{\\textrm{1h}}$ on the vacuum desiccator (1-800-4Bel-Art, Bel-Art Products) to remove any bubble present in the mixture, and cured at $70^{\\circ}\\mathrm{C}$ for $^{\\textrm{1h}}$ (Memmert $\\mathrm{GmbH}+\\mathrm{Co}$ . KG 100–800 oven). SU-8 $20055{\\upmu\\mathrm{m}}$ thickness homogeneous coating has been defined by a 2-step spin coating on previously dehydrated silicon pieces (step 1: 500 rpm for 10 s at $100\\mathrm{rpm}.s^{-1}$ , step 2: $3056\\mathrm{rpm}$ for $30s$ at $300\\mathrm{rpm}.s^{-1}$ ; Modular spin coater ws-650- 23NPP, Laurell Technologies Inc.). After a soft baking step on a hot plate ( $95^{\\circ}\\mathrm{C}$ for $2\\mathrm{min}$ , SD160 hotplate, Stuart), the SU-8 2005 has been exposed UV light (17 $s,5.95\\mathrm{W.cm}^{-2}$ , UH-H 254, UV Light Technology LTD; black filter: $320{\\mathrm{-}}405{\\mathrm{nm}},$ ), followed by another soft bake step $95^{\\circ}\\mathrm{C}$ for $3\\mathrm{min}^{\\cdot}$ ) and finally left to cool to room temperature. All steps have been prepared inside a laminar flow hood (Faster-BSC-EN) to avoid surface contamination",
        "category": " Materials and methods"
    },
    {
        "id": 9,
        "chunk": "# 2.3. Surface wettability modification process – CVD  \n\nThe chemical composition of the surface layer was varied by exposing the surface to a volume of $6{\\upmu\\mathrm{L}}$ HMDS or FDTS vapor on a vacuum desiccator (Bel-Art Products) for 2, 10, 20, 30 or $50\\mathrm{min}$ at ambient temperature $(22^{\\circ}\\mathrm{C})$ and pressure - $-0.78\\ \\pm\\ 0.09$ atm (R5 rotary vane vacuum pump, Busch) (Fig. 1a).",
        "category": " Materials and methods"
    },
    {
        "id": 10,
        "chunk": "# 2.4. Sessile static drop method  \n\nContact angle analysis (Fig. 1b) has been performed dispensing $6{\\upmu\\mathrm{L}}$ liquid drops of DI water or PBS solution on the surface with a controllable syringe pump (NE 4000, New Era) and $1\\mathrm{mL}$ syringe (CODAN) plus polyethylene tubbing BTPE-90 ${863.3\\upmu\\mathrm{m}}$ inner diameter, Instech Lab). CA measurements have also been performed in cleaned, non-exposed control sample surfaces referring to exposure time $0\\mathrm{min}$ . Images of each droplet on the surface (Fig. 1c) have been taken under ambient conditions within $5\\mathrm{min}$ after CVD exposure, unless otherwise stated. The effectiveness and stability of SWM over time has been evaluated through CA measurements over $65\\mathrm{h}$ after CVD. Droplet images have been recorded with a CMOS camera $(5.1\\upmu\\mathrm{m}$ pixel size, 12 Mpixel) coupling a macro lens with 0.33 maximum magnification. Each combination of experimental conditions (substrate – liquid – CVD fluid – exposure time, see Table 2) has been repeated three times. CA evaluation has been performed using Image $\\boldsymbol{\\mathrm{~J~}}$ software with LBADSA plugin.",
        "category": " Materials and methods"
    },
    {
        "id": 11,
        "chunk": "# 3. Experimental results and discussion  \n\nThe properties of liquid water are strongly influenced by a variety of cohesive internal interactions in liquid water molecules: van der Waals forces, dipole interactions, hydrogen bonds and proton exchange. In wettable surfaces (hydrophilic surfaces), the forces associated with surface-liquid interaction are greater than the cohesive forces in water molecules and the liquid water spreads on the surface. If internal cohesive forces dominate over surface-liquid interaction forces, drops are formed on the surface [37].  \n\nSurface wettability modification has been characterized by measuring the contact angle of sessile drops of liquid in contact with rigid surfaces. The surfaces have been previously exposed to chemical treatment according to process parameters depicted in Table 2. Random contact angle errors have been determined with a precision of $\\pm\\:\\%$ pixel by assessing the CA using LBADSA plugin of one single image 20 consecutive times. From this evaluation an error of $\\pm\\:2^{\\circ}$ was obtained. In their study, Williams and co-authors found an error of $-1.1^{\\circ}$ using the same methodology [38].  \n\nResults depicted in Fig. 2 show HMDS chemistry is more effective for SWM of highly wettable surfaces $\\mathrm{(CA}<20^{\\circ}\\$ into moderately wettable surfaces $(\\mathrm{CA}^{\\sim}70^{\\circ})$ after $30\\mathrm{min}$ chemical exposure. CA on both silicon-containing surfaces (glass and silicon surfaces), initially hydrophilic $\\mathbf{\\tilde{C}A}\\cong17^{\\circ})$ , showed an increase after chemical exposure of the surface to HMDS, which indicates a decrease in surface interfacial energy. The surface coverage seems to reach a plateau after $30\\mathrm{min}$ of CVD exposure to HMDS as the CA reaches a plateau of $\\mathrm{CA}_{\\mathrm{glass,t}30}=65^{\\circ}$ and $\\mathrm{CA}_{\\mathrm{Si},\\mathrm{t}30}=73^{\\circ}$ whereupon preserved.  \n\nWhen in the presence of HMDS, the oxygen of hydroxyl groups $(-\\mathrm{OH})$ on water-free silicon-containing surfaces will chemically bond to Si atoms of HMDS molecule, accompanied by the release of ammonia $\\left(-\\mathrm{NH}_{3}\\right)$ (Fig. 3). Additionally to the hydrophobic nature of the trimethylsilyl tail group $(-\\mathrm{{Si}}({\\mathrm{CH}}_{3})_{3})$ , the surface interfacial energy is dependent on the extent of surface coverage, unreacted residual groups from the silane on the surface and distribution and orientation of these specific functional groups grafted on the surface [41].  \n\n![](images/ba3c81e0490554000eee3001942c6fa98f6bdc2788fd5c8a3f1134db2ca9cc94.jpg)  \nFig. 1. Experimental apparatus. a) CVD setup used to chemically modify surface wettability, b) contact angle measurement apparatus, c) example of image captured for CA analysis.  \n\nTable 2 Experimental conditions tested to determine the influence of process parameters in surface wettability modification (SWM).   \n\n\n<html><body><table><tr><td>Rigid surface</td><td>Liquid used in CA analysis</td><td>Fluid used for SWM CVD exposure time [min]</td><td></td></tr><tr><td>Glass slide</td><td>DI water</td><td>HMDS,</td><td>FDTS,</td></tr><tr><td>Silicon piece</td><td></td><td>0,2,10,20,30,50 min,</td><td>0,2,10,20,30,50 min,</td></tr><tr><td>PDMS membrane</td><td>PBS</td><td>p = -0.78 ± 0.09 atm,</td><td>p = -0.78 ± 0.09 atm,</td></tr><tr><td>SU-8 2005 coating</td><td></td><td>Tamb = 22 °℃</td><td>Tamb = 22 °℃</td></tr></table></body></html>  \n\nDI water and PBS drops on SU-8 surfaces, initially with CASU-8,w,t0 $=112^{\\circ}$ , showed a decrease in CA after activation with HMDS (variation from $120^{\\circ}$ to $100^{\\circ}.$ ) throughout the different activation times, reaching a plateau after $30\\mathrm{min}$ of activation. The decrease in CA is believed to originate from the reaction between epoxide groups present in SU-8 and secondary amines of HMDS (Fig. 4). The free electrons of the nucleophile atom from HMDS (in this case, the nitrogen) react with the methylene group $\\left(-\\mathsf{C H}_{2}\\right)$ of the epoxide to obtain a less hydrophobic hydroxyl group $(-\\mathrm{OH})$ on SU-8 surface [42].  \n\nThe additional hydrogen atoms in SU-8 surface after activation coming from the hydroxyl group $(-\\mathrm{OH})$ increases the number of hydrogen bonds and consequently the forces of interaction between the surface and the liquid drop, leading to a decrease in CA. However, the surface-liquid interaction forces are not sufficient to overcome the cohesive forces of bulk liquid water, hence the surface maintains its hydrophobic behaviour (CA $100^{\\circ}.$ [37].  \n\nZisman described the influence of hydrogen atoms on surface energy, by modifying fluorocarbon surfaces $\\left(\\mathrm{CF}_{3}\\right)$ with hydrogen atoms $(\\mathrm{CF}_{2}\\mathrm{H})$ [43]. The author observed increasing surface energy in the order: $\\mathrm{CF}_{3}<\\mathrm{CF}_{2}\\mathrm{H}<\\mathrm{CH}_{3}<\\mathrm{CH}_{2}$ . Here, the modification of one single atom of fluorine for an atom of hydrogen duplicated the surface energy, leading to higher surface wettability. Likewise, surface-liquid interaction forces on surfaces containing $-\\mathrm{CH}_{3}$ groups are expected to be weaker than those containing hydroxyl groups $(-\\mathrm{OH})$ or hydrogen atoms $(-\\mathrm{H})$ . PDMS is a polymer with exposed methyl $\\left(-\\mathsf{C H}_{3}\\right)$ groups whose reactiveness also depends on the adjacent substituents (Fig. 5). In the case of PDMS, these groups are very unreactive and the attack from the nucleophile atom of HDMS does not occur. Hence, the CA values measured on PDMS surfaces are seen not to significantly change after HDMS CVD exposure. Nonetheless, this strategy for SWM of PDMS surfaces can be pursued to prevent adhesion between the PDMS master and a PDMS mold, as example [12,45,46] or act as self-cleaning layers [47]. Overall, the variation of chemical composition at the surface enabled to modify the surface contact angle never exceeding $120^{\\circ}$ , well in accordance with Terpilowski and Goncharuk [48].  \n\nPerfluorodecyltrichlorosilane (FDTS) molecules, like HDMS molecules, form self-assembled monolayers (SAM) in which the Si from trichlorosilane functional groups $(\\mathrm{-}{\\mathrm{{SiCl}}_{3}})$ covalently bonds to oxide surfaces to release hydrochloric acid (HCl) (Fig. 6) [27]. The decrease on wetting behaviour of silicon and glass surfaces observed from the increase in CA from $20^{\\circ}$ to around $70^{\\circ}$ after FDTS activation in Fig. 7a  \n\n![](images/2b68755c2df4d59c45d9d04c7b96e2561dc8d271413a5f57d70540d86a48ca8e.jpg)  \nFig. 2. Contact angle of a) DI water droplets and b) PBS droplets on rigid surfaces of glass, silicon, SU-8 and PDMS after CVD using HMDS, c) examples of static contact angles obtained with $6{\\upmu\\mathrm{L}}$ sessile droplets of liquid water on glass, silicon, SU-8 2005 and PDMS surfaces.  \n\nV. Silverio, et al.  \n\n![](images/867c8985b56da53607272d4f432f89db634c40f4c99cfda4dcc9567843ac10d9.jpg)  \nFig. 3. Reaction between HMDS and a silicon-containing surface. –Si atoms in HMDS chemically bond to the oxygen of hydroxyl groups, accompanied by the release of ammonia. (adapted from [39]).  \n\n![](images/bf4336848b86067611cfa3dbd1190188da2b109483496656973dfb2182240c89.jpg)  \n\n![](images/e9a6718c658c68b6b123e751d6b3cb6d11384dd03d95201cb8d98764cd2ac7f3.jpg)  \nFig. 4. Reaction between HMDS secondary amine and the epoxide group of SU-8 photoresist. The nucleophile atom of HMDS (nitrogen) reacts with the methylene present in the epoxide group of SU-8, forming an hydroxyl group. (adapted from [40]).  \n\nfor water and Fig. 7b for PBS, is attributed to the heavily fluorinated hydrophobic tail group of the SAM at the surface. The surface coverage seems to be reached later for glass surfaces (saturation around $50\\mathrm{min}$ FDTS CVD exposure) when compared to silicon surfaces (saturation around $20\\mathrm{min}$ FDTS CVD exposure) probably due to the boron ions in the composition of glass, which reduce the reaction kinetics of FDTS with hydroxyl groups at the surface [49].  \n\nCA on modified PDMS surfaces showed a slight increase after $2\\mathrm{min}$ CVD exposure with FDTS while on SU-8 modified surfaces present a slight decrease. As seen before, the increase or decrease in surface hydrophobicity by changing the surface chemical composition is limited by the wettable nature of the new chemical groups on the surface. The polimerization step of SU-8 monomers after post-exposure bake opens the epoxide group of SU-8 forming an hydroxyl group at SU-8 surface (Fig. 8a), available to react with the Si from the triclorosilane group of FDTS (Fig. 8b) which may explain the slight decrease observed in CA.  \n\n![](images/90cf00d19d52c01beebcc2b9523de88b5ddf20dc6de724b561214ab0ee1feb38.jpg)  \nFig. 5. Molecular structure of PDMS. Here n is the number of monomer repetitions. (adapted from [44]).   \nFig. 6. Reaction between FDTS (a self-assembled monolayer reagent) and a silicon surface. (adapted from [50]).  \n\n![](images/9df31606799b0f44c566ed6d6db172cac151759f5116b0a80b40ffe904b74836.jpg)  \nFig. 7. Contact angle of $6{\\upmu\\mathrm{L}}$ liquid droplets of a) DI water and b) PBS onto glass, silicon, SU-8 and PDMS surfaces after CVD exposure with FDTS.  \n\n![](images/b10319d8dd09e52f2196406eccb7d015f02c08f2c3c0b8412683818d3a19eb11.jpg)  \nFig. 8. Polimerization of SU-8 monomers in post-exposure bake (adapted from [51]). b) Reaction between FDTS and the SU-8 photoresist after post-exposure bake. The Si from the triclorosilane group of FDTS reacts with the oxigen of SU-8 releasing HCl.  \n\n![](images/4623ee197a63983a07f86c543e4a5e6ef8093ac02b4ac201dfcd5288e65d834d.jpg)  \nFig. 9. Variation of CA between a silicon surface and droplets of DI water and PBS buffer, after $50\\mathrm{min}$ of CVD exposure with a) HMDS and b) FDTS. In both cases after $65\\mathrm{h}$ the CA remains around $70^{\\circ}$ , which indicates a prolonged effect of chemical activation. Contact angles of DI water on Si surfaces modified with c) HMDS and d) FDTS.  \n\nThe quantification of surface chemical composition and structure can be realised recurring to techniques such as XPS, ToF-SIMS, XRF, LDFTMS or OES [52,53]. Vandencasteele and co-authors [54] used XPS to correlate the surface modification of PTFE, PVDF and PVF with nitrogen and oxygen plasma with measurements of contact angle of water droplets onto the same materials. Contact angle was directly correlated with polymer defluorination, corroborated by XPS analysis. This analysis becomes even more important when multiple layers of different materials or alloys need to contact to fabricate the final device. As example, Jung et al. [55] used XPS to correlate the effectiveness of graphene adhesion between silicon dioxide and glass, by modified anodic bonding with measurements of contact angle. The results show the contact angles obtained are directly dependent of the density of C–O bonds. Analysis techniques such those presented above are powerful tools for process validation in the several stages of microfabrication.  \n\nThe stability of SWM is a crucial parameter to successfully implement surface modification strategies in the fabrication and usage of microfluidic devices. The effectiveness of SWM over time has been evaluated for a silicon surface after $50\\mathrm{min}$ activation with HMDS (Fig. 9a) and FDTS (Fig. 9b). The results presented in Fig. 9a for DI water $\\mathrm{CA}_{\\mathrm{Si,HMDS,H2O}}=63\\pm5^{\\circ})$ and PBS $\\mathrm{CA}_{\\mathrm{Si,HMDS,PBS}}=63\\pm4^{\\circ})$ on the silicon surface show negligible variation of CA over time after SWM with HDMS whereas FDTS modified surfaces (Fig. 9b) present slightly higher variation in CA measured $\\mathrm{CA_{Si,FDTS,H2O}}=72\\pm8^{\\circ}$ and $\\mathrm{CA_{Si,FDTS,PBS}}=73\\pm7^{\\circ}$ .  \n\n![](images/8dcfc06d3cda6ec076dcfa631db7c494d17e2745b6bd8eb90bad4ecd4041e15f.jpg)  \nFig. 10. Self-assembled monolayers (SAM) of FDTS in the presence of water vapor in the reaction chamber. The presence of water promotes the formation of compact SAM by means of strong oxygen bonds. (adapted from [57]).  \n\nThis slight variability in surface contact angle may be due to lowerquality FDTS SAM on the surface originated by the absence of water vapor inside the desiccator. Although Si–Cl groups are strongly reactive with silanol functional groups (–SiOH), it is unlikely that all three terminal chlorine atoms of the same FDTS molecule react with the irregularly located silanol groups on the Si surface. As so, the SAM contains molecules that may not be firmly attached to the surface resulting in low-coverage or low-quality coverage of the Si surface [56]. To work around this limitation, Zhuang and co-authors [27] used water vapor in the reaction chamber to promote SWM with FDTS (Fig. 10). A prior reaction of −OH groups in water and –SiCl3 groups in FDTS allowed obtaining a denser and more stable self-assembled monolayer. In fact, compared with other more expensive and complex surface modification techniques (see Table 1), the simple yet very reliable chemical modification strategy pursued in this work using HMDS or FDTS on silicon surfaces as showed consistency over long periods of time over $65\\mathrm{h}$ . Due to the simplicity of this approach and the small experimental scheme needed, it can be easily implemented in diverse research and industrial scenarios.",
        "category": " Results and discussion"
    },
    {
        "id": 12,
        "chunk": "# 4. Conclusions  \n\nIn this work, chemical modification of surfaces of silicon, glass, SU-8 photoresist and PDMS was tested to understand the effect of HMDS and FDTS on the surface wettability. The experiments showed that the efficiency and stability of surface coverage are highly dependent on the availability of $-s\\mathrm{iOH}$ groups at the surface. Surface wettability after CVD modification was perceived to be governed by the wettable nature of the tail group deposited at the surface.  \n\nCA measurements of drops of DI water and PBS buffer on the surfaces activated with HMDS shows that surface energy of hydrophilic surfaces (glass and silicon) decreased with the increment of activation time. In other words, CA increased with activation (CA variation from $20^{\\circ}$ at $\\mathbf{t}=0\\mathrm{min}$ to $60^{\\circ}/70^{\\circ}$ at $\\mathbf{t}\\geq30\\mathrm{min},$ , reaching a plateau after $30\\mathrm{min}$ of activation. SU-8 photoresist, which is a hydrophobic surface, showed a decrease in CA, reaching a plateau after $30\\mathrm{min}$ of activation.  \n\nFor SWM with FDTS, the results obtained are similar those obtained with HMDS: both glass and silicon showed an increase in CA (variation from $20^{\\circ}$ to $60^{\\circ}/75^{\\circ})$ . The highest CA measured for glass was between 30 and $50\\mathrm{min}$ of activation, while for silicon, the highest CA was measured between 20 and $30\\mathrm{min}$ of activation. SU-8 showed a slight decrease in CA after $2\\mathrm{min}$ of activation (variation from $120^{\\circ}$ to 98°).  \n\nPDMS shows no relevant variation after activation with HMDS nor with FDTS.  \n\nThe evaluation of SWM persistence after $50\\mathrm{min}$ chemical activation of the Si surface with HMDS and FDTS shows the bonds formed from the reaction between Si and HMDS is more stable than those between Si and FDTS. FDTS shows a high potential to decrease surface energy due to its heavily fluorinated tail group but the absence of water vapor in the experiments leads to low-quality SAM growth.  \n\nThis surface wettability modification approach is seen as a simple and cost-efficient method to effectively control wettability of surfaces also attractive for industrial environments.",
        "category": " Conclusions"
    },
    {
        "id": 13,
        "chunk": "# Conflicts of interest  \n\nThe authors have no competing interests to declare.",
        "category": " Results and discussion"
    },
    {
        "id": 14,
        "chunk": "# Acknowledgements  \n\nINESC-MN acknowledges Fundação para a Ciência e a Tecnologia (FCT) funding through the Instituto de Nanociência e Nanotecnologia (IN) Associated Laboratory and projects POCI-01-0145-FEDER-016623 and PTDC/CTM-NAN/3146/2014, financed by FEDER (Quadro Portugal 2020) and FCT. This project has received funding from National Funds through FCT under the project PTDC-FIS-PLA-31055- 2017.",
        "category": " Acknowledgements"
    },
    {
        "id": 15,
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