Antiwetting Fabric Produced by a Combination of Layer-by-Layer

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Antiwetting Fabric Produced by a Combination of Layer-by-Layer Assembly and Electrophoretic Deposition of Hydrophobic Nanoparticles Young Soo Joung and Cullen R. Buie* Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States Downloaded by SWINBURNE UNIV OF TECHNOLOGY on September 5, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acsami.5b05233

S Supporting Information *

ABSTRACT: This work describes a nanoparticle coating method to produce durable antiwetting polyester fabric. Electrophoretic deposition is used for fast modification of polyester fabric with silica nanoparticles embedded in polymeric networks for high durability coatings. Typically, electrophoretic deposition (EPD) is utilized on electrically conductive substrates due to its dependence on an applied electrical field. EPD on nonconductive materials has been attempted but are limited by weak adhesion, cracks, and other irregularities. To resolve these issues, we coat polyester fabric with thin polymer layers using electrostatic self-assembly (layer-by-layer self-assembly). Next, silica nanoparticles are uniformly dispersed on the polymer layers. Finally, polymerically stabilized silica nanoparticles are deposited by EPD on the fabric, followed by heat treatment. The modified fabric shows high static contact angle and low contact angle hysteresis, while keeping its original color, flexibility, and air permeability. During a skin fiction resistance test, the hydrophobicity of the coating layer was maintained over 500 h. Furthermore, we also show that this approach facilitates patterned regions of wettability by modifying the electric field in EPD. KEYWORDS: electrophoretic, layer-by-layer, antiwetting, patterned-wettability, fabric coating

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INTRODUCTION Cost-effective, scalable methods to create antiwetting fabric are in high demand for applications ranging from outerwear to water purification and oil−water separation.1−3 Furthermore, promising fabrication methods can be employed for developing other types of functional fabric including antimicrobial, UV-absorbing, fire retardant, and conductive.4−7 Given the numerous potential applications, many coating techniques have been reported for functionalizing fabric.8,9 Among these methods, the use of nanoparticles has been accepted as the most promising because various types of functionality can be achieved with different nanoparticles.6,7,10−15 Furthermore, the use of nanoparticles is effective in maintaining the original fabric appearance, flexibility, and permeability after coating. Various coating methods have been investigated to implement nanoparticle-based-coatings for antiwetting fabric. Spray coating, bar-coating, and deep-coating are considered simple methods, but it is challenging to obtain durable and uniform coatings and to control wettability.7,16−19 Surface functionalization utilizing chemical vapor deposition or sol−gel processes have shown good hydrophobicity, but the intrinsic characteristics of the fabric are more likely to be distorted during processing.20−24 Recently, electrostatic assembly techniques (layer-by-layer self-assembly, LBL) have been investigated, demonstrating uniform superhydrophobic fabrics.25,26 Advantages of this technique include coating uniformity and precise control of coating thickness. LBL is effective in controlling nanoscale coating thickness because the © XXXX American Chemical Society

thickness is linearly proportional to the number of LBL cycles. LBL employs electrostatic attraction between particles or between particles and substrates.27 When two surfaces have opposite charges, they can be self-assembled if the electrostatic force is sufficient to overcome the other repulsive forces. Using polymers or surfactants, which have high absolute values of the zeta-potential, surface charges of nanomaterials can be enhanced.28,29 Due to its precise thickness control, LBL has been utilized to produce thin functional films with various nanomaterials.29−31 However, electrostatic assembly is limited by its long processing times and increased manufacturing costs. Furthermore, it is challenging to utilize LBL to produce thin films with unstable suspensions containing large particles with low surface charge. This limitation is due to fast sedimentation and insufficient electrostatic forces on particles with low surface charge. Another challenge in coating nanoparticles on fabric is to deposit with spatial selectivity. In previous studies, patterned superhydrophilic and superhydrophobic surfaces produced with silicon nanowires have been used to investigate selective adhesion and transfer of biomolecules and nanoparticles, in addition to mass spectrometry of biomolecules.32−34 Further, in the past decade patterned superhydrophobic surfaces have been Received: June 14, 2015 Accepted: August 27, 2015

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DOI: 10.1021/acsami.5b05233 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Downloaded by SWINBURNE UNIV OF TECHNOLOGY on September 5, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acsami.5b05233

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION Surface charge is necessary for electrostatic self-assembly and electrophoresis, and the amount of charge is estimated by zetapotential measurements. Nonaqueous solvents are necessary to make stable suspensions of hydrophobic nanoparticles. In this work, we used methanol for the solvent of PDMS modified SiO2 (PDMS-SiO2) particles and polyvinylidene fluoride (PVDF) because methanol has low viscosity, which results in high deposition rates.49 Antiwetting fabric can also be produced by EPD with various kinds of nonaqueous solvents such as acetone, isopropanol, and ethanol; therefore, methanol toxicity can be avoided by using one of the alternative solvents. Figure 1a shows

investigated to enhance the efficiency of harvesting water from fog.35,36 To maximize efficiency, the patterns and wetting properties must be optimized. Conventional microfabrication techniques including photolithography,37,38 polymer dewetting,39 selective deposition of chemicals and polymers,40,41 spatial photo-cross-linking,36,42 and patterned imprinting43,44 have been employed to produce patterned superhydrophobic surfaces. However, there have been few attempts to produce patterned antiwetting fabric because conventional microfabrication techniques are not effective to apply to soft and permeable materials. Therefore, new fabrication techniques are necessary to implement patterned wettability. Technology for patterned antiwetting fabric can also be utilized for other purposes such as conductive fabrics with customized electric circuits.10,45 In our previous work, superhydrophobic surfaces were produced by electrophoretic deposition (EPD) with polydimethylsiloxane (PDMS) modified SiO2 nanoparticles (PDMSSiO2) on steel plates.46 This method provides fast and customizable deposition of superhydrophobic surface coatings. The coating thickness can be controlled by the electric field intensity and deposition time. Furthermore, the morphology on the modified surface can be altered by changing the suspension stability during EPD.46 Additionally, the durability was dramatically enhanced by codeposition of nanoparticles with polymeric adhesives. EPD has tremendous potential to overcome the drawbacks of existing methods for coating fabric with nanoparticles. However, to date most EPD processes have been employed on electrically conductive substrates due to the necessity of electric field lines perpendicular to the deposition substrate in order to achieve sufficient adhesion.47−49 Some studies have explored the possibility for EPD on electrically nonconductive substrates.50,51 Among these investigations, ionic conductive films and porous substrates show deposits after EPD.52−54 While fabrics are a promising candidate for EPD because they are permeable to solvents and can be penetrated by electric fields after swelling, weak adhesion, poor deposition quality, and limited control of deposition thickness have plagued previous attempts due to the high electric fields employed.55−57 Generally it is accepted that higher electric fields produce less uniform coating layers.58 The high electric fields result in significant irregularity because concentration polarization near the substrate results in large, weakly adhered nanoparticle agglomerates on the nonconducting substrate. In this work, we introduce a hybrid method employing LBL and EPD for coating nanoparticles on fabric. EPD and LBL have been independently investigated to develop antiwetting surfaces,46,59,60 but to best of our knowledge this work is the first report on a hybrid method employing EPD and LBL for antiwetting surfaces. Using the hybrid method, we can produce durable, uniform, and patterned antiwetting fabric. The resulting material possesses the original properties of the fabric such as color, flexibility, and breathability. First, using LBL, thin polymerlayers are deposited and then covered by a uniform dispersion of nanoparticles. The first polymer layer enhances the adhesion between nanoparticles and the substrate after heat treatment. The initial layer of nanoparticles provides deposition nucleation sites, preventing aggregation on the substrate during EPD.46,61 Next, EPD is conducted with polymer stabilized nanoparticle suspensions to achieve thick deposited films of nanoparticles. The suspension is stable enough to yield uniform deposition on the substrate due to the steric repulsion between nanoparticles. Further, hydrophobic regions can be spatially patterned on fabric modified by EPD using spatially varying electric fields.

Figure 1. Zeta-potentials of the polymers and the SiO2 nanoparticles used in this study. (a) Zeta-potentials of poly(sodium 4-styrenesulfonate) (PSS), poly(diallyldimethyldiammonium chloride) (PDDA), SiO2 nanoparticles with respect to pH. (b) The table summarizes the zeta-potentials at pH 8.5. The zeta-potential of untreated polyester fiber was obtained from a reference.64 The solvent for the SiO2 nanoparticle and PVDF suspensions is composed of methanol 90 vol % and water 10 vol %. The other solvents are all deionized water. Concentrations are 8 g/L PSS and PDDA and 1 g/L SiO2 and 0.5 g/L PVDF with and without 3.75 × 10−3 g/L PDDA polymers. The error bars indicate ±1 std dev from three measurements.

the zeta-potentials of the suspensions of PDMS-SiO2 nanoparticles and polymers, used in this work, with respect to pH. The zeta-potentials of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethyldiammonium chloride) (PDDA) solutions (8 g/L for each) are −25 mV and +20 mV at pH 8.5, respectively (Figure 1b). Due to the high absolute zeta-potentials of PSS and PDDA, both polymers have been extensively used for LBL.62,63 The zeta-potentials of PDMS-SiO2 nanoparticles and PVDF, dispersed in the methanol (90 vol %) and water (10 vol %) mixtures, were −25 mV and −19 mV without PDDA polymers, and +10 mV and +24 mV with 3.75 × 10−3 g/L concentration of PDDA polymers, respectively (Figure 1b). The sign change of the zeta-potential of PDMS-SiO2 and PVDF indicates the formation of PDMS-SiO2-PDDA and PVDF-PDDA assemblies. Because PDMS-SiO2 and PVDF have the same sign of zetaB


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Figure 2. Schematic illustrations of the fabrication process suggested for antiwetting fabric coating. (a) Step 1: multilayered polymers are electrostatically deposited on the fabric. Step 2: nanoparticle are electrostatically deposited on the polymer layers of the fabric. Step 3: the polymer and nanoparticle assemblies are deposited by electrophoretic deposition (EPD). Step 4: heat treatment is conducted on the fabric for enhancing mechanical durability. (b) The EPD cell for Step 3 in (a). The cell consists of two stainless steel electrodes with the fabric substrate wrapped around the cathode electrode.

zeta-potentials become lower than in the aqueous suspensions, resulting in insufficient electrostatic attraction. For example, PDDA has a zeta-potential of −8.18 mV with standard deviation of 63.3 mV and PSS solidified in methanol. Furthermore, the top layer of the hydrophobic nanoparticles prevents self-assembly of polymers dispersed in aqueous suspensions. Conversely, sufficient film thickness can be achieved by EPD due to the relatively high electrophoretic force compared to the electrostatic force. The use of EPD has additional advantages to produce antiwetting fabric; first, wettability is a function of deposition time, and second, we can directly use unstable suspensions, which have low zeta-potential or large particles, for EPD due to the high electrophoretic force and the fast processing time. In addition, the modified surface area is entirely dependent on the electrode size and the electrolyte volume during EPD. Therefore, the fabrication method is easily scalable to large areas of fabric. We used cylindrical shaped steel electrodes during the entire fabrication process (Figure 2b). We expect that continuous roll processes could be applied to this method. Fabrics can be immersed in polymer solutions for pretreatment, and then using rolling steel electrodes, nanoparticles can be coated on the fabric. Because the suspensions of polymers and nanoparticles can be cycled, the hybrid method of LBL and EPD would be costeffective. To enhance mechanical durability of the modified fabric, annealing processes are conducted at temperatures slightly lower than the melting points of the polymers employed after EPD. The melting temperatures of polyester, PSS, PDDA, and PVDF are >230 °C, 460 °C, ∼ 300 °C (decomposition temperature), and 177 °C, respectively. Because PVDF has a melting temperature lower than that of polyester, PVDF provides good adhesion between nanoparticles and fabric after the annealing process. While both PVDF and PDMS-SiO2 are hydrophobic materials, PDMS-SiO2 particles are smaller and have lower surface energy; therefore, the single use of PVDF is not effective at producing highly antiwetting fabric. Indeed, fabric coated with only PVDF showed maximum static contact angle less than 130°.

potential, both can be codeposited on the same electrode with EPD. For PDMS as a reference, the polyester fibers have a zetapotential of −45 mV at pH 8.5 in water, and the isoelectric point (IEP) is pH 2.3.64 Based on the zeta-potentials measured, the fabrication process, as shown in Figure 2a, is suggested. PDDA naturally deposits on the polyester fabric due to electrostatic attraction because polyester and PDDA have negative and positive surface charge, respectively. Then, PDDA and PSS layers can be produced by alternating immersions of each polymer solution due to their opposite surface charge. Next, immersing the polyester fabric pretreated by PDDA and PSS into a suspension with 1.0 g/L PDMS-SiO2 and 0.5 g/L polyvinylidene fluoride (PVDF) leads to self-assembly of the nanoparticles on the PDDA. Finally, on the PDMS-SiO2-PVDF layer, PVDF-PDDA-PDMS-SiO2 assemblies are uniformly deposited by EPD with the suspension composed of 1.0 g/L PDMS-SiO2, 0.5 g/L PVDF, and 3.75 × 10−3 g/L PDDA. The concentration of PDMS-SiO2 was decided by considering deposition rate and suspension stability. With higher concentration, faster deposition rates can be obtained; however, the suspension becomes unstable due to the increase in particle−particle interactions. Based on our experimental results, 1.0 g/L PDMS-SiO2 concentration provides sufficient deposition rate and with a relatively stable suspension, simultaneously. To determine the concentration of PVDF, mechanical durability and hydrophobicity of the deposited films were considered. With the higher PVDF concentration, higher mechanical durability was achieved after the annealing process but the antiwetting property was reduced because PVDF is less hydrophobic than PDMSSiO2. As a result, when the concentrations of PDMS-SiO2 and PVDF are 1 g/L and 0.5 g/L, highly durable and antiwetting fabric can be obtained. The use of EPD leads to rapid fabrication, much faster than the multiple immersions required for thick electrostatic assemblies. We found that it is challenging to produce coatings of polymers and hydrophobic nanoparticles with LBL because they are dispersed in aqueous and nonaqueous suspensions, respectively. When the polymers are dispersed in nonaqueous solvents, the C


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of PSS−PDDA enhanced the absorbance significantly (Figure 3a). After the first two bilayers, the absorbance of the polyester film increases with increasing numbers of PSS−PDDA bilayers (Figure 3b). Because, as shown in Figure 3c, PDMS-SiO2 has the high turbidity and PDDA has lower absorbance than PSS,65 the absorbance enhancement is mainly attributed by PDMS-SiO2 and PSS in (a) and (b), respectively; therefore, the absorbance change with respect to the number of the bilayers indicates that PSS and PDDA were in fact deposited on the polyester films by LBL. Through the fabrication process suggested, we produced antiwetting fabric coated by PDMS-SiO2 nanoparticles, as shown in Figure 4a. Even though the original fabric instantly absorbed


When PDMS-SiO2 particles were solely used for EPD, the maximum static contact angle exceeds 150° but the mechanical durability was very low due to the weak adhesion between the particles, even after annealing. Therefore, we can achieve antiwetting fabric with good mechanical durability utilizing a codeposition of PVDF and PDMS-SiO2. UV−vis spectroscopy has been used to confirm the bilayers of PDDA−PSS produced by LBL.65 Figure 3 shows UV−vis spectra (absorbance change) of polyester films, with different numbers of PSS and PDDA layers. The first two bilayers of PSS−PDDA reduced the absorbance in the wavelength range of 400−700 nm because thin polymer films improve antireflection of transparent films.66 The single layer of SiO2 nanoparticles on the two bilayers

Figure 4. Images of water droplets on polyester fabrics modified by the fabrication process shown in Figure 2. Two different fabrics, which color red and brown, were used to produce antiwetting fabrics. Electrophoretic deposition was conducted for 90 s in Step 3. (a) The modified fabric does not lose its original color but the droplets show high contact angles and low roll-off angles. Flexibility of the original fabric is maintained after modification while wettability changes dramatically from hydrophilic to superhydrophobic. (b) A water droplet is rapidly absorbed into the original polyester fabric. The scale bars indicate 1 mm. (c) The brown fabric does not show significant color change before and after modification. Water droplets show contact angles higher than 150° and roll-off angles lower than 5°. (see Supplementary Movies S1−S6).

water droplets (Figure 4b), droplets on the modified fabric show static contact angles higher than 150° and roll-off angles less than 2° when EPD was conducted for 90 s. The modified fabric is still flexible with minimal color change (Figure 4a and c) (see Supplementary Movies S1−S6). This method results in uniform antiwetting performance on the entire area modified. The fabric wettability can be altered by the duration of EPD. Static contact angle and roll-off angle were measured on the fabric produced as a function of deposition time during EPD

Figure 3. UV−vis spectra (absorbance change with respect to wavelength) of polyester films with respect to the number of polymer layers assembled by layer-by-layer (LBL). (a) The absorbance decreases with increasing number of PSS and PDDA deposition, up to the first two-bilayers. (b) The absorbance increases with the number of LBL after the third round of deposition. (c) Digital image of suspensions of PDMS-SiO2, polyvinylidene fluoride (PVDF), PDDA, and PSS. D


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Figure 5. Static contact angle and roll-off angle of fabric produced with different deposition times for EPD. Contact angles were measured after heat treatment. (a) Water droplets were instantly absorbed into the original fabric, resulting in the effective static contact angle of zero degrees. The static contact angle is 136° on the fabric coated with polymer and thin layers of hydrophobic SiO2 nanoparticles (after Step 2). Static contact angle is dramatically enhanced after EPD process (after Step 4). The maximum contact angle of 159° and the minimum roll-off angle of 2° are achieved at 90 s deposition time. The error bars indicate ±1 std dev from five measurements. (b) Contact angles on the antiwetting fabric with respect to solution pH. The contact angle on the fabric is not significantly affected by droplet pH. In the inset, all droplets, which have different pH values, have contact angles greater than 155° on the fabric. The colors of the droplets indicate solution pH; red, transparent, green, and blue indicate pH 4.0, 5.5 (DI water), 7.0, and 10.0, respectively. (c) Contact angles of fabric coated with PDMS modified silica nanoparticles before and after the chemical stability tests. The coated fabrics were immerged into different pH water containers, which were pH 1.0, deionized (DI) water, and pH 10.0, for 80 h. After the fabrics were fully dried, the static contact angles were measured. The coated fabric had the average static contact angle of 152. 2° before the test.

(155°) and a lower roll-off angle (5°) than before the annealing process (153° and 12°). Color change was investigated with the deposited film thickness by comparing the RGB values of the coated fabric with respect to the duration of EPD, as shown in Figure 7. As shown in Figure 7a, the film thickness is linearly proportional to the duration of EPD in the initial deposition region; however, the rate of increase becomes slower due to the electrical resistance of the deposited film.48 As shown in Figure 7a, the current density decreased with the increase of the film thickness, indicating the deposited layers have higher electrical resistance than the particle suspension. We also measured current density with respect to the thickness of fabric layers on the working electrode. Three different thickness fabrics were used to compare the electric resistance in the EPD system. As shown in Figure 7b, the current density increased with the increase of the fabric thickness. As a result, the fabric layers, immerged into the suspension, have higher electric conductivity than the particle suspension. Due to the higher electric field, the film thickness, obtained at the same EPD time, increased with the increase of fabric thickness, as shown in Figure 7c. From comparison between the film thickness and the color change, an optimal EPD time can be determined to maintain the original color with sufficient antiwetting. The relative RGB values (R*, G*, and B*) are the ratios of the RGB values at a specific EPD time to the original fabric (Figure 7d). It is known that thin SiO2 nanoparticle layers, uniformly deposited, are highly transparent.67−69 For instance, Cebeci et al. showed that transparent superhydrophilic glass can be achieved using thin layers of SiO2 nanoparticles.67 In this work, the fabric colors also did not change significantly even after the static contact angle with water reaches 150° (Figure 7e). It is worth noting that the dense and uniform SiO2 nanoparticles were deposited on polyester fibers with the help of the SiO2 nanoparticles and polymer films coated prior to EPD.

(Figure 5). The contact angle of water droplets on the fabric before EPD (time is zero in Figure 5) was approximately 130°. This shows that the electrostatic assembly was not sufficient to achieve an antiwetting surface. After EPD, the static contact angles were significantly enhanced and low contact angle hysteresis was achieved. With increasing EPD time, the static contact angle increased and the roll-off angle decreases. The maximum static contact angle and minimum roll-off angle were achieved at 90 s deposition time. A similar relationship between the contact angle and deposition time was found in our previous work on steel plates.46 To understand the effect of droplet pH on antiwettability, contact angles were measured with buffer solutions with different pH (3.0, 7.0, and 10.0) on an antiwetting surface, which has a contact angle of 158° with DI water. The contact angles exceeded 155° for all the pH values without a significant difference, as shown in Figure 5b. In addition, to check the chemical stability of the coating layers, we have measured the contact angles 80 h after the antiwetting fabrics were immerged into strong acid and base solutions, for which pH values were 1.0 and 10.0, respectively. The contact angles did not decrease after the acid and base solution tests as shown in Figure 5c. As a result, the coating layers, composed of the silica nanoparticles and polymers, have sufficient chemical stability to be used in a wide range of pH from 1.0 to 10.0. The morphology of the modified fabric was investigated using SEM images, as shown in Figure 6. The original fabric has a twill wave pattern composed of 15 μm polyester fibers (Figure 6a). After Step 2 in Figure 2, SiO2 nanoparticles are uniformly deposited on the fabric (the inset of Figure 6a). The coatings on the fabric become thicker with increasing duration of EPD (Figure 6b,c) after Step 3. The SiO2 particles are partially deposited on the fibers after 30 s of EPD (Figure 6b) but fully covered after 60 s of EPD (Figure 6c−f). The resulting antiwetting fabric shows a slightly higher static contact angle E


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Figure 6. SEM images of fabrics produced by electrophoretic deposition (EPD) with different deposition times. (a) Original polyester fabric has twill weave patterns. The inset shows a fabric fiber with SiO2 nanoparticles deposited on the surface (after Step 2 in Figure 2). (b) After 30 s of EPD, SiO2 nanoparticle layers are observed on the fabric but do not fully cover the surface. (c) After 60 s of EPD, the fabric fibers are uniformly and densely coated with SiO2 nanoparticles. This surface exhibits the maximum contact angle of 155° and minimum roll-off angle less than 5°. (d−f) Magnified SEM images of the fabrics after 60 s of EPD.

To evaluate the breathability of the antiwetting fabric, water vapor transmission (WVT) rates were measured according to the standard method for water vapor transmission of materials (ASTM E 96, Procedure B, 1999).70 The WVT rate can be calculated as WVT = ΔM/(A·t), where ΔM is the weight change of liquid water, A is the fabric surface area, and t is the time elapsed to obtain ΔM. We filled 80 mL of liquid water into 100 mL media bottles and capped the bottles with vented caps sealed with fabric having 9.1 × 10−3 m2 test-area. The WVT rates were measured every 5 h and the average WVT rates were calculated from five measurements at a specific water temperature. The water temperature was controlled from 35 to 65 °C at intervals of 10 °C. The test environmental conditions were 24 ± 1 °C and 30 ± 5% relative humidity with negligible air flows on the fabric during testing. Interestingly, slightly higher WVT rates were obtained from the antiwetting fabric than the original fabric across the entire temperature range. The difference becomes greater at higher temperatures, as shown in Figure 8. At the least we can say that the coating does not reduce breathability but the data suggests it may be enhanced. We hypothesize that the hydrophobic nanoparticles can prevent condensation, enhancing direct vapor transport through voids of the fabric.

Durability of the fabric coatings was evaluated by a skin friction resistance test using boundary layer flow. A circulating pump continuously generated water flows at 10 cm/s on the coating layer attached to a steel plate. We measured the static contact angle with respect to test time on the fabric. The fabric was fully dried on a hot plate at 50 °C and cooled down to the environment temperature before the measurement. Figure 9 shows the change of the contact angle on the antiwetting fabric during the test. The fabric maintained contact angles higher than 155° over 500 h. Even though the static contact angle decreased, the coatings were not completely eroded. Future work will involve further enhancements to the fabrication process in order to maintain superhydrophobicity in harsh environments. One advantage of utilizing EPD for fabric coatings is the ability to produce patterned surfaces. We demonstrate this by producing patterned antiwetting surfaces using honeycombshaped electrodes (Figure 10a). As shown in Figure 5a, the fabric wettability can vary with the duration of EPD. In addition, the electric field during EPD can be utilized to control local wettability.46 When shaped electrodes are used, the electric field on the fabric varies spatially. The weakest electric fields are induced at the center of the holes while the highest electric field F


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Figure 7. Color tests using RGB comparison and real images of the modified fabric. (a) Thickness of the deposited films on polyester fabric and current density at the electric field of 60 V/cm with respect to duration of EPD. The thickness is linearly proportional to deposition time in the initial deposition region. The current density decreases with the increase of the film thickness. (b) Current density change during electrophoretic deposition (EPD) with different numbers of fabric layers. The constant voltage of 120 V was applied during EPD and the thickness of fabric was changed with the number of fabric layers. (c) The average film thickness deposited with 500 s EPD time with respect to the number of fabric layers. (d) R*, G*, and B*, which are the RGB values relative to the original fabric, with respect to deposition time during electrophoretic deposition (EPD). The error bars indicate ±1 std dev from five measurements. The RGB values were obtained by using image software (Adobe Photoshop CS4) with captured images of fabric produced with varying EPD time. (e) Color change of fabrics produced at different EPD times.

Figure 9. Contact angles on the antiwetting fabric with respect to test time. The durability test was conducted with a customized skin friction generator. The skin friction was induced by boundary layers on the fabric with flows circulated by a water pump. The flow speed was 10 cm/ s. The contact angles were measured after the fabric was fully dried. Each data point is the average contact angle. The roll-of angles were estimated with the advancing and receding contact angles. The error bars indicate ±1 std dev from five measurements.

Figure 8. Water vapor transmission (WVT) rate tests for the original fabric (box symbols) and the antiwetting fabric (circle symbols). The antiwetting fabric was produced with 90 s of EPD at 60 V/cm. The WVT rate experiments were conducted with water temperatures ranging from 35 to 65 °C. The error bars indicate ±1 std dev from four measurements.

occurs at the steel features. The sample fabric coated using the shaped electrode is shown in Figure 10. Even though the fabric has nonuniform deposits on its surface, there is no discernible

color difference on the surface. To test the locally varying wettability, water was sprayed on the fabric at a speed of 0.6 mL/ G


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Figure 10. Patterned antiwetting fabric using a patterned electrode. (a) An electrode with honeycomb shaped holes (left) was used to coat hydrophobic nanoparticles on polyester fabric (right). The fabric coated by the patterned electrode does not show significant color difference relative to the original fabric. (b−d) Water was continuously sprayed on two pieces of fabric, one with spatially homogeneous antiwetting and a second with patterned antiwetting, and the resulting droplets were imaged with respect to time. (b) Droplets grow irregularly on the uniform antiwetting fabric. (c) Droplets grow with regular patterns and ultimately show similar patterns as the electrode. (d) Droplet coalescence on the patterned antiwetting fabric (the top and right inset) is similar to dew drops on a leaf (the bottom and left inset). (see SI Movie S7).

cm2·min. The water spray results in droplets growing with time as they merge with the spray droplets (Figure 10b,c). Interestingly, the growing droplets have regular patterns similar to the patterns of the electrode holes (Figure 10c,d) (see SI Movie S7). This result shows that the electric field using customized electrodes can be used to alter the fabric wettability. This allows us to control fabric characteristics with respect to location as well as deposition time. In nature, one can observe regular distribution of dew drops on leaves, as shown in the inset of Figure 10d. The uniform patterns prevent the mass concentration caused by large drops and the subsequent deflection of the surface. This same feature can be obtained on the antiwetting surfaces with regularly patterned wettability.

Water droplets on the modified polyester fabrics show static contact angles exceeding 150° and contact angle hysteresis less than 2°. SEM images of the fabric show uniformly and densely deposited nanoparticles on the polyester fibers. An optimal EPD time of 90 s was shown to produce highly antiwetting fabric without significant color distortion. The antiwetting fabric maintained its hydrophobicity over 500 h during an aggressive skin friction resistance test. Furthermore, fabric wettability can be controlled by EPD time and electrode geometry. We expect that this method can be directly employed to develop commercial functionalized fabric. Large areas can be modified by our method in a time and cost-effective manner. In addition, the resultant fabric has sufficient durability for practical applications, but more work must be conducted for more aggressive applications. Perhaps the most unique advantage of this method is that patterns of wettability can be implemented using varying electrode geometries. Considering these advantages, we believe that this method can be widely utilized to functionalize fabric with nanoparticles for various purposes.

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CONCLUSIONS We have presented a coating method employing electrophoretic deposition to create antiwetting polyester fabric. Advantages of this method include of scalability, durability, and control of wettability, showing great potential for commercial use. These features have been achieved by a hybrid method employing electrostatic assembly and electrophoretic deposition for coating three different constituents (polymers, nanoparticles, and polymer-coated nanoparticles) on fabric surfaces. The first polymer layer and the second nanoparticle layer are electrostatically deposited on the fabric surface. The first polymer layer provides strong mechanical networks between the fabric and the polymer-coated nanoparticles after heat treatment. The second nanoparticle layer provides nucleation sites for the subsequent EPD. The polymer-coated nanoparticles show low zeta-potential but they were highly stable in the suspension because of steric repulsion between particles. As a result, this method provides advantages of both EPD and electrostatic deposition: this method is fast, cost-effective, and scalable while also capable of precise control.

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EXPERIMENTAL SECTION Materials. We used two kinds of color polyester fabric (Patagonia, Fabric A: brown, 100% polyester, 20 denier in warp and 20 denier in fill direction, and Fabric B: red, 100% polyester texturized fiber without any coating/treatment, 10 CFM in air porous, and plain weave face and twill weave back). Two kinds of polymers, poly(diallyldimethyldiammonium chloride) (PDDA) (Aldrich, 30 wt % solution in water, molecular weight (MW) < 100,000) and poly(sodium 4-styrenesulfonate) (PSS) (Aldrich, 35 wt % solution in water, MW = 70,000), were used as positively and negatively charged polymers, respectively, for the electrostatic coating process. Both polymer concentrations were 8 g/L in deionized (DI) water. These two polymers have been widely used for layer-by-layer techniques because they can be electrically charged in dielectric solvents, resulting in high electrostatic H


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ACS Applied Materials & Interfaces attractive and repulsive forces.30,31 Two kinds of suspensions were prepared; in the first suspension: PDMS-SiO2 (PlasmaChem, 14 nm) and polyvinylidene fluoride (PVDF, SigmaAldrich, MW = 534,000) are dispersed into a mixture of methanol (90 vol %) and water (10 vol %) with concentrations of 1 and 0.5 g/L, respectively; and in the second suspension: PDDA is added to the SiO2 and PVDF suspension described above until the PDDA concentration reaches 3.75 × 10−3 g/L. Finally, the pH of both suspensions is adjusted to 8.5 with a basic solution (potassium hydroxide, 45% for HPLC, Sigma-Aldrich). Electrophoretic Deposition. The fabrication process for antiwetting fabric consists of four steps, as shown in Figure 2. First, a steel plate (type 316 stainless steel foil, full hard temper, 0.004 in thickness, 10 cm × 30 cm, Trinity Brand Industries, Inc.) is wrapped with a sheet of fabric (20 cm × 30 cm). In this step, slightly curved steel plates make good contact with the fabric by providing a small amount of tension. Next, the steel plate and fabric assembly is immersed into the PDDA solution (8 g/L concentration in DI water) for 1 min and then the fabric is washed with DI water for 10 s. Afterward, the assembly is immersed into the PSS solution (8 g/L concentration in DI water) for 1 min and then washed with DI water for 10 s. These two immersion-washing steps are conducted twice (Step 1). Next, the fabric-plate assembly is immersed into the nanoparticle suspension (the first suspension) for 5 min and then washed with DI water for 20 s (Step 2). For EPD in the second suspension (Step 3), the fabric-electrode assembly and a similarly sized stainless steel plate are used as the cathode and the anode, respectively. During EPD, an electric potential difference of 120 V is applied to the electrodes, which are separated by 20 mm. After EPD, the fabric is dried in the laboratory atmosphere, and then heat-treated at 150−220 °C for 2 min with a heat-press apparatus (Clamshell, Model PRO-3804X) (Step 4). To produce patterned antiwetting fabric, hexagonally patterned perforated sheets (McMaster-Carr, hole diameter 0.25 in., open area 79%, center-to-center spacing 0.281 in., and thickness 0.032 in.) were used as the electrodes for EPD, as shown in Figure 10a. Material Characterization. Zeta-potential of polymers and nanoparticles dispersed in solution were measured by a zetapotential meter (Zetasizer Nano ZS, Malvern Instruments, Inc.). Two scanning electron microscopes (SEM, JEOL 6320FV FieldEmission High-resolution SEM and He-Ion microscope, Zeiss) were used to observe the morphologies of modified fabric surfaces. Static and dynamic contact angles were measured with a goniometer (Kyowa, DM-CE1) with a 3 μL drop of DI water. Water droplet behaviors on the fabric were recorded using a digital camera (J2, Nikon). Color changes before/after coating were evaluated by RGB comparison with an image processing program (Adobe photoshop CS4). A customized skin friction generator was used for durability tests.

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Uniform vs patterned antiwetting fabric (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y.S.J. and C.R.B. designed the research; Y.S.J. performed the research; Y.S.J. and C.R.B. analyzed the data and wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding was provided by Battelle Memorial Institute. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-08-19762. We thank T. Ohara for providing the fabric material.

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ABBREVIATIONS EPD, electrophoretic deposition; PDMS, polydimethylsiloxane; WVT, Water vapor transmission; PDDA, poly(diallyldimethyldiammonium chloride); PSS, poly(sodium 4-styrenesulfonate); DI, deionized; PVDF, polyvinylidene fluoride; SEM, scanning electron microscopes; LBL, layer-by-layer self assembly

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b05233. Original vs MIT-modified fabric as in Figure 4a (AVI) Droplet absorption as in Figure 4b (AVI) Original vs modified fabric under water spray as in Figure 4c (AVI) Droplet coagulation (AVI) Droplet rolloff (AVI) Droplet rolloff under water faucet (AVI) I


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Antiwetting Fabric Produced by a Combination of Layer-by-Layer

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