Surface & Coatings Technology 254 (2014) 230–237

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Surface & Coatings Technology

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Bio-inspired polydimethylsiloxane-functionalized silicaparticles - epoxy bilayer as a robust superhydrophobic surface coating

Aleksander Cholewinski 1, Josh Trinidad 1, Brendan McDonald, Boxin Zhao ⁎University of Waterloo, Department of Chemical Engineering and Waterloo Institute for Nanotechnology, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada

⁎ Corresponding author.E-mail addresses: aacholew@uwaterloo.ca (A. Cholew

(J. Trinidad), brendan.mcdonald@uwaterloo.ca (B. McDon(B. Zhao).

1 Equal contribution.

http://dx.doi.org/10.1016/j.surfcoat.2014.06.0200257-8972/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 December 2013Accepted in revised form 12 June 2014Available online 21 June 2014

Keywords:SuperhydrophobicityRobustnessSilica particlesEpoxy layerDip coatingEmulsionBio-inspiration

A robust superhydrophobic bilayer coating is developed containing polydimethylsiloxane (PDMS)-functionalizedsilica particles on top and an epoxy bonding layer at the base. It is fabricated with a facile dip-coating processthat embeds micron-scale PDMS-functionalized silica particles with nano-scale roughness into an epoxy layerspin-coated onto a substrate. The dip-coating process uses multiple cycles to generate a repeatable and consistentcoating onto a glass substrate. The resulting bilayer coating is able to be applied tomultiple surfaceswhere the con-ventional epoxy coating is used. The concentration of silica particles was systematically varied, showing minimalinfluence on hydrophobicity and mechanical properties. The robustness of the coating is characterized byobserving the wear properties and integrity of the rough surface as it is scratched with a stainless steel probe,while adhesive tape was applied repeatedly to the surface to observe any modification to the hydrophobicity.Additionally, a different substrate material, a transparent flexible polymer, has been successfully coated. Thisworkdemonstrates a facile dip-coating process using commercially available silicamicro/nanoparticles to generatea robust superhydrophobic coatingwith the potential for a larger scale application to improve on the performanceof conventional epoxy coatings.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, the fabrication of superhydrophobic surfaces has be-come an interesting subject in many research activities to meet the in-creasing need of self-cleaning and waterproof advanced materials[1–13]. This subject draws inspiration from the natural example of thelotus leaf, a plant that possesses a waxy substance with low surface en-ergy and a hierarchical surface structure at the micro- and nanoscopiclevels [1–4]. The surface of the lotus leaf is composed of cilium-likenanostructures on top of micro-scale papillae [4–6], which has becomethe model for constructing superhydrophobic surfaces [2,3,7]. Thesestructures have the ability to elicit a superhydrophobic effect, that is, asurface that exhibits water contact angles above 150° [4–6,8–12], aswell as a contact angle hysteresis below 10° [5,8,13]. Superhydrophobicsurfaces have a low adhesion to water, causing the beading of waterdroplets upon contact with the surface. This property can induce a self-cleaning effect [2,6,8,14,15] that allows the water droplet to carry awaydirt and contaminants at low inclined angles [6,14].

inski), ejtrinid@uwaterloo.caald), zhaob@uwaterloo.ca

The general process of producing a hydrophobic surface usingmicro/nano particles involves either (a) creating a rough surface by ad-dition of particles via coating methods and then functionalizing therough surface [7,9,15] or (b) producing hydrophobic particles first andthen adding these particles onto the surface using coating methods[3]. Our work follows the latter method and aims to mitigate the inher-ent issue with synthetic superhydrophobic surfaces, that is, the weakmechanical properties that these surfaces exhibit [9,12]. This character-istic weak surface strength is also observed with the lotus leaf model, aslight physical contact with the hierarchical structures of the lotus leafleads to damage of the micro/nano-structures, resulting in a loss ofsurface properties [8,9,12,14].

In this work, we present the development of a robust super-hydrophobic bilayer coating containing polydimethylsiloxane (PDMS)-functionalized silica particles on top and an epoxy bonding layer at thebase. It is fabricated with a facile dip-coating process that embedsmicron-scale PDMS-functionalized silica micro/nanoparticles into anepoxy layer spin-coated onto a substrate. Unlike some methods thatare spray-based [16–18], the proposed method in this paper is designedto embed rough particles (to provide hydrophobicity) onto a conven-tional epoxy coating layer (to provide mechanical strength) which canbe readily coated on a variety of substrates. The experimental resultsshowed that this bilayer coating remains sturdy and functional when ex-posed to external stress so it will not require frequent re-applications,

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ultimately cutting costs and adding beneficial engineering properties toconventional epoxy coatings.

2. Materials and methods

2.1. Materials

The HDK H18Wacker silica (PDMSmodified/functionalized) hydro-phobic nanoparticles were kindly donated by Wacker Chemie. Theparticle size of HDK H18 PDMS-silica particles is in the range of 0.1–1 μm in diameter, with an average diameter of 0.5 μm. These are madefrom individual silica spheres, 5–30 nm in size, which are fused togetherat high temperatures. These particles also form some agglomerateshaving a size of 1–250 μm. Dow epoxy resin, D.E.R. 331 (2,2-bis(p-(2,3-epoxypropoxy)phenyl)propane), and Dow epoxy curing agent,D.E.H. 24, were purchased from Dow Chemical Company. 99% tetrahy-drofuran (THF) solvent, 99% HPLC methanol and 99% isopropanol(IPA) were purchased from Sigma Aldrich. All chemicals were used asreceived. Ultra-pure H2O was dispensed from a Direct-Q Millipore Ap-paratus. The glass slides (Fisher Finest, Premium Plain GlassMicroscopeSlides 1″ × 3″) were purchased from Fisher Scientific. These were cutdown to 1.5″ in length. The flexible substrate Xerox Removable StripeTransparencies (referred to as flexible film), were purchased fromXerox. Both glass and flexible film substrates were pre-cleaned ina bath of methanol and dried by gentle wiping with Kim-wipes. Thetape used for testing the surface of the coating was aluminum ducttape (which was determined to be hydrophobic with an averagewater contact angle of around 113°).

2.2. Dip-coating emulsion

A batch reactor was used to make the dip coating emulsion byadding a mixture of HDK H18 particles and THF into 70 mL of ultra-pure H2O. The HDK H18 particles were mixed with THF in a 20 mLglass vial. Weight percentages of the particles in THF ranged from 0.5%to 4% w/w (used 20 mL of THF) and from 6% to 8% (used 10 mL ofTHF). The mixture was vortexed for 1 min to ensure a good dispersionwhich was then poured into ultra-pure water in the jar and vortexedagain until the formation of a cloudy white emulsion. The emulsioncontained in the open jar was placed onto a hotplate at 125 °C toallow the particles to move up to the top layer, which is necessary forthe success of the coating.

2.3. Preparation of epoxy layer

Epoxy resin (D.E.R. 331) and curing agent (D.E.H. 24) were used inthis procedure at a ratio of 13:1 resin to curing agent (as opposed tothe fully-curing 100:13 ratio). This was to slow down the curing of theepoxy during the iterative procedure, allowing it to hold onto the parti-cles during dip coating, while keeping the particles from sinking fullyinto the epoxy and losing surface roughness. To ensure a well-mixedepoxy solution, the resin was vortexed for at least 5 min upon additionof the curing agent. In order to have a uniform coating surface, theepoxy/curing agent solution was put in a desiccator to degas undervacuum for a minimum of 5 min. After degassing, the epoxy/curingagent was diluted with IPA at a ratio of 30 μL of IPA per gram of resinand vortexed for 2 min to reduce the viscosity. The diluted epoxy solu-tion was then spin-coated onto the substrate at 3700 RPM for 40 s.

2.4. Fabrication of the superhydrophobic coating

After being spin-coated with epoxy, the substrate was dipped usingtweezers into the heated emulsion in the jar placed on the hotplate. Thesubstrate was swayed back and forthwithin the foam layer at the top ofthe jar, then slowly withdrawn at an approximate angle of 45° andplaced onto the hotplate face-up. The glass jar was then removed from

the hotplate to be vortexed until the liquid appeared to be cloudywhite again (~20 s), then returned onto the hotplate. The coated sub-strate was placed into a glass Petri dish filled with THF with the coatedside facing down; the back of the substrate was tapped with the twee-zers. The sample was then swirled within the THF to get rid of theloose particles and was gently pulled out of the Petri dish. Afterwards,the coated substrate rested on a paper towel in a fume hood for ~30 s,and was then placed on the hotplate for ~20 s. After this, the previoussteps were repeated for another two to three times. At the end of thefinal iteration, the sample was left on the hot plate for 20 min to curethe epoxy.

2.5. Characterization methods

Thewater contact angles were determined from silhouettes of static30 μL droplets, using an elliptical best-fit model. The water contactangles were taken from the top-center, center-center, and right-centerof the glass slide. Each snapshot contained two contact angles, givingsix contact angles per sample. Static, advancing and receding contactangles were measured by placing 9 μL water on the surface, addingadditional 9 μL water and then withdrawing the extra water using asyringe pump at 1.3 μL/s with the needle kept in the droplet. To deter-mine the surface structure of the coated sample and obtain a topograph-ical image of the surface, an optical profiler (Rtec Instruments) wasused. Data from the topographical image was processed using theGwyddion image analysis software in order to obtain RMS roughnessvalues. Friction and wear tests were carried out using a UniversalMaterial Tester (UMT) Tribological Test Equipment (CETR Campbell,CA, USA), with a hemispherical steel probe (R = 0.5 mm). For eachfriction test, the probe was slid over a 4 mm distance under a constantload for four half-cycles. The friction test was performed at variedloads in increments of 0.5 N over a range of 0.5 to 5 N on differentsections of the surface.

3. Results and discussion

3.1. Coating PDMS-functionalized nanoparticles

Fig. 1 illustrates the coating process that utilizes PDMS-functionalized silica particles to generate a hierarchical roughnesswith a low surface energy material. Similar work in producingsuperhydrophobic surfaces via embedding of nanoparticles onto thesurface is presented by C. Su et al., where they employed a methodinvolving the spin coating of the epoxy and ethanol suspended silicananoparticles, followed by a functionalization process [7]. Compara-tively, our coating approach has two unique features. One is the useof an unstable emulsion of the PDMS-functionalized nanoparticlesfor coating process; the other is theuse ofwet epoxypre-layer to enhancethe bonding strength of the nanoparticles intended to be embedded onthe surface.

It was experimentally determined that simply drop-coating a sus-pension of the HDKH18 particles in an organic solvent was not effectivedue to the aggregation of the hydrophobic particles during drying.Therefore, a dip-coatingmethodwas developed using an emulsion solu-tion,which served as a batch reactor. Unlike emulsion solutions used formaking latex particles [16], or spray-on solutions [16–18], this emulsionis unstable; a thick foamy top layer is formed when heated at 125 °C,which is able to prevent particle aggregation. The water has two roles:the first is to act as a medium to aid in the generation of foam, and thesecond is to ensure that the particles stay within the upper layer foreasier dipping since the hydrophobic particles move away from thewatery phase. THF serves as a good solvent for PDMS, allowing the func-tionalized particles to dissolve in the required weight percentage. Also,according to J. Lee et al., THF has the 7th highest swelling ratio for PDMSamong the solvents they examined. Higher swelling indicates highersolubility (the solubility parameters of the two components are similar

Fig. 1. A schematic of the iterative dip-coating process from base components to the final coating. The number of dipping iterations is a function of the concentration of HDK H18 and thevolume of H18/THF solution.

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in value), and ensures better suspension of particles [19]. Furthermore,its boiling point of 66 °C aids in its evaporation on a surface to leave theparticles behind [20], allowing for simple working conditions. Withthese factors in mind, THF was selected as the best solvent to use forsuspending the particles. Fig. 2 illustrates the typical appearance ofthe prepared emulsion showing two separated phases: the top foamylayer and the lower watery layer. There are also some particles at thebottomof the jarwhich could not go to the top layer at this temperature.Note that the phase separation is necessary for the coating process. Weattempted to coat the substrate using only the foamy layer after remov-ing the watery phase but this resulted in failure primarily due to thefoam not being able to spread itself uniformly onto the substrate. Wealso attempted to coat the substrate using only the watery phase afterremoving the top foam layer. It also resulted in failure because therewere not sufficient particles in the watery phase. Preliminary tests indi-cate that the optimal hotplate pre-heating temperature of 125 °C is

Fig. 2.A) the chemical structure of the PDMS functional group bonded to the silica particles. B) Awere fused together before functionalization to form the aggregate. C) An illustration of an agdiameter of 0.5 μm. D) The agglomerates of the nanoparticle aggregates, which ranged from 1 tthe foamy top layer and the watery phase. This unstable emulsion is formed after multiple itercoating using 2 wt.% silica/THF depicting the structure and size of particles (B), aggregates (C)

necessary for keeping the THF/HDK particles floating above the water.Lower temperatures cause the THF/particles mixture to sink to the bot-tom of the jar, while higher temperatures would cure the epoxy andevaporate the THF too rapidly. Furthermore, the temperature allowsfor the gradual curing of the epoxy pre-layer, and was experimentallydetermined to be low enough to not deform the flexible film whenusing it as the substrate. The temperature of 125 °C is kept constantthroughout the procedure.

After creating the nanoparticle emulsion, epoxy resin and curingagent are mixed in a 13:1 ratio, and then desiccated to remove airbubbles. IPA is included to reduce viscosity of the epoxy for easier spincoating. The epoxy-spin coated substrate is dipped into the batch reac-tor using narrow-nosed tweezers (to reduce the contact made with thesurface). During this process, the HDK H18 particles sit on top of theuncured epoxy but partially buried into the liquid. Once the substrateis submerged within the foam layer, it is then swayed back and forth

schematic of a silica nanoparticle that was functionalizedwith PDMS. These nanoparticlesgregate. These aggregates have a size distribution from 0.1 μm to 1 μm, with an averageo 250 μm in size. E) A photo of the batch emulsion reactor containing two separate phases:ations of vortexing and heating. F) A closer view of the foam layer. G) A SEM image of theand agglomerates (D).

Fig. 4. The water contact angle on the silica-embedded epoxy bilayer coated surface as afunction of the concentration of HDK H18 particles in THF without tape application andwith two or four tape applications. Error bars are the standard deviation of the contactangles from at least five repeating tests.

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to maximize particle attachment onto the surface and slowly retractedfrom the hotplate. To ensure that the process yields optimal results,the substrate's surface must possess numerous small bubbles asopposed to fewer, larger ones. Next, the substrate is placed onto thehotplate face-up to partially cure the epoxy and evaporate the THF.The batch reactor is then removed from the hotplate, sealed tightlywith its lid and then vortexed until all of the contents are visiblyswirling. This mixing is required to regenerate the foam's uniformity, asdipping the substrate creates a holewithin the layer of THF/nanoparticles.The batch reactor is then placed on top of the hotplate (lid-off) to allowthe particles to aggregate and float upwards. Next, the substrate is placedface down into a THF filled 20 mL glass Petri dish to remove the excessfoam. The substrate (faced down) is lightly tapped all over the substratearea and then flipped over and swirled in THF to remove loose particles.Next, the substrate is gently pulled out of the Petri dish, then allowedto rest on a paper towel for ~30 s (inside the fume hood). The vacuumof the fume hood allows for partial evaporation of the THF. Next,the slide is placed onto the hotplate to further evaporate the THF. It isexpected that the particles would become visible in a cloudy whitenessto the surface after the second iteration, as the THFwill evaporate almostinstantly. This process is repeated three to four times until finally, thecoating is left on the hot plate for ~20 min to allow the epoxy to fullycure. The same coatingmethodwas also successfully applied to a flexiblefilm substrate.

3.2. Superhydrophobicity of the coated substrates

Water contact angle measurements were performed to characterizethe hydrophobicity of the coated surfaces. Fig. 3 illustrates the waterdroplets on coated glass and flexible film substrates. Both possessa water contact angle greater than 150°, indicating their super-hydrophobic states. There are two wettingmechanisms used to explainthe high contact angles made possible by modifying a surface: theWenzel wetting state, and the Cassie–Baxter wetting state (where theliquid droplet sits on top, but does not wet the entire surface) [21,22].Two factors contribute to the creation of hydrophobic surfaces: both alow surface energy (such as fluorinated or silanized surfaces) and theaddition of roughness to a surface enhance the wetting properties.Adding roughness can be achieved by physically marking the surface,etching (chemically or via plasma), or by adding micro/nanoparticlesonto the surface through some coating procedures [1,11,15,21,23,24].In particular, a hierarchical roughness and a low surface energy areoften required together for a superhydrophobic surface [3,11]. The coat-ing procedure developed in this work uses PDMS-functionalized silicaparticles to both reduce surface energy (fromPDMS) and add roughness(from the particle structure). To identify the wetting state, qualitativeand quantitative tests were carried out. The goal was to determineif the coating would bead off water droplets (using a small tilt angle),as the Cassie–Baxter state predicts a surface that has not only a highcontact angle, but also a low contact angle hysteresis, allowing waterdroplets to bead and roll off the surface [22]. Both coated glass and

Fig. 3. Depiction of the coating applied to a flexible film substrate (left) and a glass substrate (riangle.

flexible film substrates were able to successfully bead off the particles,indicating that the observed superhydrophobicity is in the Cassie–Baxter state. Furthermore, the contact angle hysteresis, determined asthe difference between advancing and receding contact angles, wasfound to be negligible (b2°) for all coatings that contained particles,confirming that the surface has low hysteresis and in the Cassie–Baxterstate.

3.3. Effects of nanoparticle concentration on the hydrophobicity

Various journal articles such as the one written by Karmouch andRoss take interest in the impact that concentration has on the qualityof the coating [17]. However, the aim of this article is not to find anoptimal concentration, but rather to determine if there is a differencein the performance and quality of the coatingwhen varying the concen-trations of the PDMS-functionalized silica particles. In this experiment,the concentration of HDK H18 within the THF solution increased inincrements of 2% each time (starting at 0% and ending at 8%) with theexception of 0.5%, which was used to verify if a low concentrationremains functional. Fig. 4 illustrates the stark difference between thesamples coated with particles compared to those without (i.e. onlythe epoxy pre-layer), with a much lesser effect from changing theconcentration. The data provides evidence that superhydrophobicity isachieved when varying the concentrations within the range of 0.5%to 8% w/w.

From Fig. 4, it is interesting to note that the contact angle slightlydecreased as the concentration was increased past 4%. We suspectedthat the gradual drop is due to the particles aggregating past a critical

ght). Note that the static contact angles are similar, and both surfaces exhibit a low sliding

Fig. 5. Advancing, receding, and static water contact angles on the silica-embedded epoxycoated surface as a function of concentration of HDK H18 particles in THF. Advancingand receding angles were obtained by injecting and withdrawing the water, respectively,from a droplet. Error bars are the standard deviation of the contact angles from at leastthree repeating tests.

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concentration, thus leading to the decrease of the micro-roughness/micro-structures formed. In addition to measuring static contact angles,hysteresis values were determined using advancing and recedingcontact angles. The resulting measurements for advancing, receding,and static contact angles can be seen in Fig. 5. The three types of contactangles have very similar values, resulting in negligible hysteresis values(b2°). The samples were further examined using SEM. The resultingimages for 2%, 4%, 6%, and 8% coatings can be seen in Fig. 6. It can beseen that the surface coverage by the silica particles increases with

Fig. 6. SEM images of the silica-embedded epoxy coated surfaces seen under 100×magnificconcentration coated surface; B) a 4% concentration coated surface; C) a 6% concentration coat

concentration, reaching a full coveragewith the 6% coating. The 2% coat-ing, in particular, has larger regions that are mostly devoid of particles.The 4% coating possesses fewer bare regions than the 2% coating, butdoes not show a complete surface coverage.

Themicro-scale roughness for the coated samples was examined byan optical profilometer, which is commonly used in the field [25,26].The roughness parameter estimated was root-mean-square roughness,Rq, whichwas calculated through Gwyddion image analysis software. Rqis defined as

Rq ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1n

Xni¼1

H−Hi

� �2;

vuut

where Hi is the height value for a point and H is the average surfaceheight [25]. The root-mean-square roughness values were found to be4.32 ± 1.86 μm, 2.58 ± 1.44 μm, 6.73 ± 2.04 μm, and 7.14 ± 4.31 μm,for the 2%, 4%, 6%, and 8% concentration coatings, respectively. The aver-age surface roughness for the coatings with particles is 5.19 ± 2.41 μm(compared to an Rq value of 0.59 ± 0.19 μm for the coating of only theepoxy layer without particles). These roughness values, taken togetherwith the high contact angles and their low hysteresis, demonstratethat the resulting coated surfaces possess sufficient roughness toachieve superhydrophobicity. In addition to micro-scale roughness,sampleswere examined at higher SEMmagnifications in order to inves-tigate the presence of nano-scale features. Images of particle aggregatestructures in 2% and 6% coatings can be seen in Fig. 7. From these SEMimages, it can be seen that both surfaces possess roughness at thenano-scale. While the bare regions can be seen in the 2% sample, theaggregates that are present also appear to have greater nano-scaleroughness than those of the 6% sample. Other concentrations wereexamined using SEM as well, and all coatings that contained particlespossessed nano-scale surface roughness. This combines with themicro-scale roughness measured with optical profilometry to form

ation using varying concentrations of silica HDK H18 particles in THF, showing: A) a 2%ed surface; and, D) an 8% concentration coated surface.

Fig. 7. SEM images of the silica-embedded epoxy coated surface at 5000×magnification demonstrating the presence of nano-scale roughness for A) a 2% concentration coated surface andB) a 6% concentration coated surface.

Fig. 8. The evolution of the contact angle from repeated tape-testing iterations for bothsilica/epoxy bilayer coated surfaces with varying silica HDK H18 particles in THF andepoxy (no particles) coated samples. Error bars are the standard deviation of the contactangles from at least five repeating tests.

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two scales of roughness and a hierarchical structure. This is the reasonfor the superhydrophobic nature of the coatings.

The coating procedure has also been tested for 0.5% w/w. It pro-duced a superhydrophobic surface. However, the problem lies inthe higher potential for variability in surface coverage (areas withless hydrophobicity). It required extra iterations with the dip coatingprocess to compensate for the decrease in uniformity. Furthermore, thislower concentration has been found to increase the difficulty in formingthe foamy layer. Overall, a practicalworking range for the concentrationwould be within 2% to 8%. Additionally, to test for reproducibility,another 8% w/w coating wasmade a month later with a freshly remadeH18/THF solution and demonstrated similar hydrophobicity. The re-sults confirm the reproducibility and general consistency of the coatingprocedure to be reliable.

3.4. Robustness of the superhydrophobic coating

3.4.1. Resistance to tape testingSimilar to thework of H. Hou and Chen [1] and J.Wang et al. [27], an

adhesive-tape test was used in an attempt to pull the particles off thesurface as a method to determine how well the coating had adheredto the substrate (and particularly, the silica particles to the epoxylayer). The main function of the particles is to deliver thesuperhydrophobicity through their PDMS functionalization and micro/nano-scale roughness. Therefore, it was determined that the best mea-surement of robustness of the coating would be to measure the watercontact angle before and after each application of tape. To ensure consis-tency, a glass jar filled with water (having a total mass of 200 g) wasrolled three times over the tape. The next step involved peeling thetape back at an angle of approximately 45° until it was fully detachedfrom the surface. This peeling was repeated with a new piece of tapeeach time until a total of four applications of tape were made. Theresults of this comparison, for each of the coating concentrations, areshown in Fig. 8. A control sample coated with only the epoxy withoutsilica particles is also shown in Fig. 8 as the dashed black line to illustratethat it is indeed the silica particles that improved the hydrophobicity.

Fig. 8 shows that for all of the glass slides with the bilayer coating,the contact angles remained constant after the tape application. Theepoxy-only coating however, showed an increase after tape application.This unexpected increase was suspected to be due to residue from thetape leaving behind some sort of hydrophobic substance on the epoxysurface. To verify, the contact angle of the tape was measured to be~113°, and found to be higher than that of bare epoxy, which is ~82°.As the contact angle of the tape is not higher than the coatings withHDK H18 particles (and is similar to the chemical hydrophobicity of

flat PDMS), it is unlikely that any tape residue should increase theirhydrophobicity.

Additional tape was applied to the samples to a total of ten applica-tions, with no further change in contact angle observed after four appli-cations. The fact that the contact angle experiences no change impliesthat the tape peeling did not damage the coating. However, we noticedthat some particles were transferred to the tape surface, suggesting thatperhaps there were some loose particles that stayed on top of the coat-ing, but were not part of the coating itself. After the tape tests, only theparticles that adhered to the epoxy layer, which are part of the coatingitself, are left behind. After a battery of tape tests, it was observed thatsuperhydrophobicity was retained, indicating that the coating is robust.

3.4.2. Resistance to friction and wear analysisFriction and wear are properties that must be understood as they

carry great importancewhen gauging the performance and life of prod-ucts that use polymers [28]. Onemethod that is commonly used to char-acterize friction andwear is sliding an indenter or probe across a surface[29,30]. The experiments conducted in thiswork involve two situations:one where the load remained at a constant value in order to determinethe wear behavior of the surface, and another situation where the loadwas progressively increased in order to view the breakdown of the sur-face. These tests were carried out for each sample with the difference

Fig. 9.Plots showing the coefficient of friction for a steel probe sliding on the coated surfaces over a range of preload forces including A)0.5N, and B)1N. The friction trace of theplot in C) isat 1.5 N following four applications of adhesive tape. D) Optical images of the scratched samples used for plots A) and B) and other scratches at higher preloads without tape applications.

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particle concentrations used in this paper. To further investigate the ro-bustness of the developed coating, a steel probe was used to apply aforce across the surface multiple times at the same spot. A change inthe coefficient of friction would indicate damage to the surface. It wasexpected that damage would be seen as a reduction in the coefficientof friction, as the rough surface of the coating would be worn away,leaving a smoother surface behind. Results of this testing can be seenin Fig. 9. At lower preloads of b1 N, the coefficient of friction does notappreciably change when the probe is slid along the surface of the coat-ing. In addition, there is no scratch visible on the surface. At higherpreloads, there is a visible scratch, as well as a noticeable change inthe coefficient of friction after the first forward half-cycle. However,for preloads of b2 N, these scratches do not pin water droplets on thesurface, and do not appear on samples that have already had four appli-cations of adhesive tape. Similarly, for these preloads as shown inFig. 9C, the coefficient of friction does not change for the post-tape sam-ples and is, itself, lower than that for the pre-tape samples, implying thatthe surface possesses less macro-scale roughness. Based on these re-sults, it is believed that these lower preloads only remove the loose silicaparticles which were not washed away by THF, leaving the durable em-bedded particles on the surface. This further supports the results fromthe tape test, where the change in contact angle became negligibleafter four applications of tape. This suggests that it is the embedded par-ticles that provide the required nano-scale roughness for asuperhydrophobic coating.

4. Conclusion

In summary, a novel method is reported involving an emulsiondip-coating procedure to provide a simple, reproducible, and me-chanically robust superhydrophobic coating for both rigid and flexi-ble substrates. In particular, this work demonstrates a method to use

commercially availablemicro/nano-silica particles for the application ofa superhydrophobic coating. The testing focused on providing com-prehensive analysis and characterization to understand the mechanicsinvolved in forming superhydrophobic surfaces, as well as understand-ing how the surface behaves and performs under mechanical stress.Water contact angle measurements were made, showing that the coat-ing is superhydrophobic with low hysteresis. The adhesion strength ofthe coating was evaluated using a combination of tape tests and contactangle measurements, where it was determined that the surface pro-duced kept the particles well rooted, with superhydrophobicity beingretained. Furthermore, wearwas determined using friction tests under-going multiple cycles and a constant load. The results of the wear testsshow that the coating layer that remained after the tape tests (thebase level) is indeed robust, just as the tape test suggested.

Acknowledgments

This work was supported by the research funds from the NaturalSciences and Engineering Research Council of Canada (NSERC) througha Discovery grant program.

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