Reinforced Superhydrophobic Coating on Silicone Rubber for

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Reinforced Superhydrophobic Coating on Silicone Rubber for Longstanding Anti-Icing Performance in Severe Conditions Alexandre M. Emelyanenko, Ludmila B. Boinovich,* Alexey A. Bezdomnikov, Elizaveta V. Chulkova, and Kirill A. Emelyanenko A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Leninsky prospect 31 bld. 4, 119071 Moscow, Russia S Supporting Information *

ABSTRACT: We present a simple method for fabricating the superhydrophobic coatings on composite silicone rubber used for electrical outdoor applications. The coating is characterized by contact angles as high as 170° and is mechanically durable in contact with the aqueous phase. We discuss the impact of mechanical durability of the surface texture on the anti-icing performance of the coating on the basis of the experimental data on freezing delay of sessile aqueous droplets. A set of complementary data obtained in laboratory and outdoor experiments on freezing delay time, variation of wettability and practical work of adhesion for supercooled aqueous sessile droplets, impacting behavior of droplets at low negative temperatures, as well as the results of snow and ice accumulation in outdoor experiments indicate the very prospective icephobic properties of the developed coating. KEYWORDS: anti-icing coating, laser treatment, surface modification, superhydrophobicity, freezing kinetics, wettability, adhesion, nanostructured surfaces

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literature5−10 where the anti-icing efficiency of certain coatings is questioned because of chemical or mechanical instability. In this paper, we will consider the anti-icing behavior of superhydrophobic coatings on composite silicone rubber used to fabricate electrical insulators for high voltage power lines. As follows from numerous literature data, the surface of silicone rubber is intrinsically hydrophobic.11,12 To reduce the interaction of an insulator surface with an aqueous medium, various types of modification leading to the superhydrophobic state of the surface were applied.13−24 In particular, in refs 16, 23, and 24 the hydrophobized SiO2 nanoparticles were deposited onto the surface of silicon rubber24 or glass electric insulator,16,23 allowing us to obtain the surfaces, characterized by contact angles of 151.0 ± 0.9°,16 161 ± 3°,23 and 164.1 ± 4.2°.24 A spin-coating of a suspension of room-temperaturevulcanized silicone rubber in hexane doped with TiO2 nanoparticles over the substrate surface allowed the authors of refs 22 to fabricate a coating with contact angle of 154.8 ± 2.1° and the roll-off angle of 6.8 ± 1.5°. Hot pressing of silicone rubber against a picosecond laser-treated steel surface, performed in ref 15, lead to achieving the superhydrophobic state of a silicone rubber with the contact angle of 151°. Modification of an outdoor silicone rubber insulator by radio frequency CF4 capacitively coupled plasma used in ref 14 allowed the authors to reach the superhydrophobic state of the

INTRODUCTION It is difficult to imagine the vital activities of contemporary society without polymers. Silicone rubber is among the most indemand synthetic polymers both in daily life and in industrial applications. This material, which is composite by nature, has good environmental resistance, allowing its use in outdoor applications where protection from weathering is critical for performance. It is the material of choice in a wide variety of transportation applications today, including automotive and aviation, in medium to high voltage electrical applications, as cable coatings and electrical insulators for power lines, and in many LED lighting applications and electrical enclosures that are exposed to outdoor conditions, due to excellent sealing properties, etc. Atmospheric icing and snow accretion are the key problems of electrical insulators used for overhead power lines, electrified railway, and municipal transport in various countries, including Russia, Canada, China, USA, and many others. The above-mentioned atmospheric phenomena cause essential disturbances in power line operation, insulation failures, power transmission poles and wire breakage, and essential power losses. In recent years, scientists and engineers have been drawn to the problem of designing passive icephobic coatings.1 The peculiar physicochemical properties of these coatings result in mitigation of snow and ice accumulation on the surface of these coatings during use in harsh outdoor conditions. Although superhydrophobic coatings are considered by the scientific community as one of the most prospective types of icephobic coatings,2−7 there are several examples in the © 2017 American Chemical Society

Received: April 20, 2017 Accepted: June 28, 2017 Published: June 28, 2017 24210

DOI: 10.1021/acsami.7b05549 ACS Appl. Mater. Interfaces 2017, 9, 24210−24219

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ACS Applied Materials & Interfaces

Figure 1. Intrinsic morphology of silicone rubber without treatment (obtained as received) (a) and the image of silicone rubber with the dimple array and water droplets (b). treatment (Bioforce Laboratories, USA) for 3 h to graft the surface hydroxyls acting as chemisorption centers. In the final step, immersion of these types of samples in a 1% solution of methoxy-{3[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-oxy]-propyl}-silane in decane for 2 h, followed by drying for 60 min in an oven at 130 °C, results in the formation of a layer of fluorooxysilane atop the lasertextured surface with multimodal roughness. We must stress the very important role of this layer of fluorooxysilane, which improves the anti-icing properties of rubber. On one hand, this improvement is due to a further decrease in surface energy since the surface energy of silicone rubber is higher than that of fluorooxysilane. On the other hand, the fluorooxysilane used in our work has three terminal functional groups, allowing it to be used as a binding agent. The reaction of one methoxy group with hydroxyl belonging to a nanoparticle located on the surface of textured silicone rubber binds a fluorooxysilane molecule to the surface, whereas the reaction of these groups with the methoxy groups of adjacent fluorooxysilane molecules leads to formation of a 2D polymer film atop the textured layer,25 which anchors the texture elements such as nanoparticles redeposited after ablation to the silicone rubber matrix. Texturing was performed at ambient conditions with T = 20−25 °C and relative humidity (RH) of 40−50%. Prior to the laser treatment, samples with sizes of 47 × 86 × 5 mm3 were ultrasonically cleaned in 95% ethyl alcohol and then in deionized water. The superhydrophobic substrates fabricated as described above show very low rolling angles (the data will be presented in the “Results” section). Thus, to prevent spontaneous droplet merging removal from the substrate in the experiments, which was performed inside an environmental chamber subject to vibrations caused by the compressor, we formed an ordered array of dimples (Figure 1b) on samples 3 and 4, with the superhydrophobic surface both within and between the dimples. The details on fabrication of these dimples can be found in an earlier paper.26 Measurement Techniques. The morphology of the sample surface and cross-section was studied using a Supra 40 VP (Carl Zeiss, Germany) field emission scanning electron microscope. An Everhart− Thornley detector was used to detect secondary electrons. To characterize the wettability of the samples, the contact angles were measured for 15 μL water droplets at five different surface locations. The measurements were performed using a homemade experimental setup27,28 for recording optical images of sessile droplets and software for digital video image processing of the droplets and subsequent determination of droplet parameters. To measure the rolling angle, 10 μL droplets were deposited on the surface. After equilibration of the initial droplet shape, controlled substrate tilting was used to obtain the rolling angle by averaging over ten different droplets on the same substrate. The practical work of adhesion of supercooled water droplets to the superhydrophobic substrates was calculated using the variation of

insulator surface with increased surface roughness and deposited fluoric groups showing the contact angles exceeding 150°. Here we present a simple and technologically attractive method for the design of superhydrophobic coatings atop a composite silicone rubber with contact angles as high as 170°. The conditions for achieving durable icephobic properties of these coatings will be discussed. Based on the experimental data on freezing delay for ensembles of sessile aqueous droplets, on variation of wettability at low negative temperatures, on the values of tensile adhesion strength of glaze ice, and on the results of snow and ice accumulation in outdoor experiments, we will show the very prospective properties of the developed coating.

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MATERIALS AND METHODS

Sample Preparation. The main object of our study is the commercial composite silicone rubber P3303 (Penta-91, Russia), fabricated by curing a mixture of polydimethylsiloxane (PDMS) and silica nanoparticles as a filling agent at high temperature and elevated pressure. The nanoparticle diameters vary in the range of 20−70 nm. The intrinsic morphology of a composite silicone rubber surface is shown in Figure 1a. In this study, we performed a comparative investigation of anti-icing behavior of samples of silicone rubber, as received (referred to below as sample 1), of a sample of silicone rubber with a chemisorbed fluorooxysilane layer (sample 2), and of two types of superhydrophobic samples (samples 3 and 4). The preparation of superhydrophobic PDMS surfaces was based on nanosecond laser treatment. In this study, we used an Argent-M laser system (Russia) with an IR ytterbium fiber laser (wavelength 1.064 μm) and a RAYLASE MS10 2-axis laser beam deflection unit (Germany). Laser treatment of the surface was performed with pulse duration of 100 ns, repetition rate of 20 kHz, and peak power of 0.95 mJ in TEM00 mode. The samples were raster scanned by the laser beam, with spot size of 40 μm, at linear speed of 250 mm s−1 with parallel line density of 20 mm−1. This treatment, accompanied by laser ablation and subsequent deposition of nanoparticles formed in the plasma onto the treated surface, leads to the formation of multimodal roughness on the aluminum alloy surface. Since silicone rubber is intrinsically hydrophobic, the roughness obtained by laser texturing with the above-mentioned parameters makes the surface superhydrophobic. The sample obtained after laser texturing of silicone rubber is referred to below as sample 3. Another type of superhydrophobic sample (referred to below as sample 4) was obtained by a three-step procedure. In the first step, the sample was laser textured with the same laser treatment parameters used for sample 3. In the second step, the sample was exposed to UV-ozone 24211


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Figure 2. SEM images of the laser-textured surface of silicone rubber at different magnifications. The sizes of the scale bar (from left to right) are 30, 10, and 1 μm.

Table 1. Wetting Parameters for the Studied Samples parameters advancing contact angle, deg. rolling angle, deg. advancing contact angle after outdoor test, deg. rolling angle after outdoor test, deg.

sample 1, nontreated silicone rubber

sample 2, silicone rubber after chemisorption of fluorooxysilane

sample 3, laser-treated silicone rubber

sample 4, silicone rubber with texture reinforcement by fluorooxysilane

112.0 ± 2.3 no rolling 127.0 ± 5.6

118.8 ± 2.5 no rolling

160.0 ± 2.5 9±3

168.0 ± 2.5 3±1 168.5 ± 0.5

no rolling

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receding contact angle and droplet surface tension when the droplet was cooled. To monitor the droplet parameters, such as the contact angle, base diameter, droplet volume, and the liquid surface tension on cooling of an individual droplet, the droplet was deposited onto the substrate inside the double-walled cell described earlier.24 To control the temperature and prevent the effect of vibrational perturbations on the measured parameters, the cell was placed on an antivibration support inside a Binder MK53 environmental chamber. The statistics of freezing for ensembles of sessile droplets on different substrates was studied using the setup developed earlier and described in detail in ref 29. Briefly, sessile droplets with the same volume of 18 μL were deposited on a substrate placed in a stainless steel cell with a transparent heat insulating cover. The cell with the substrate, the droplets, and the surrounding vapor were thermally homogeneously cooled to the required temperature over 40−55 min, and then the temperature was kept constant. The transparent cover of the cell makes it possible to detect freezing of the droplets by the change in appearance from clear to opaque using a BestDVR-405 LightNet video system. A layer of water ice on the bottom of the cell below the substrate maintains nearly saturated vapor conditions, thus preventing noticeable droplet evaporation. The time required to freeze each of the individual droplets, referred to below as the freezing delay time, was counted from the instant the desired temperature was established in the experimental chamber. The data of the experiments presented below were obtained by analysis of the freezing of 170 to 250 sessile droplets on each particular substrate at each temperature. The experiments with the different substrates were performed at T = −10 °C, −15 °C, and −18 °C. The ability of the superhydrophobic samples fabricated in this study to withstand atmospheric icing and snow accumulation was analyzed by outdoor exposure of samples of types 1 and 4 with sizes 100 × 100 mm2 from October 2016 to April 2017. The samples were mounted on a testbed with a slope of 25° to the horizontal. During this time interval, the behavior of the samples was registered by a BestDVR-405 LightNet video system at different weather impacts.

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4±1

Figure 3. Evolution of contact angle (triangles) and surface tension (circles) of a water droplet deposited on the surface of superhydrophobic sample 4 with time of contact in conditions of saturated water vapor.

length scales, and the morphology of sample 4 was very similar to that shown in Figure 2. These SEM images show the multimodal roughness obtained after laser treatment. The advancing contact angles and rolling angles measured for the studied samples are given in Table 1. Typical contact angles for all studied surfaces and rolling angles for the superhydrophobic coatings are presented in Table 1. High advancing contact angles and low rolling angles indicate the superhydrophobic state of samples 3 and 4. A comparison of wetting parameters for samples 1 and 2, on the one hand, and for samples 3 and 4, on the other hand, shows that chemisorption of fluorooxysilane on both the hydrophobic and the superhydrophobic samples does indeed make surfaces more water repellent with a higher contact angle and lower rolling angles (for superhydrophobic samples). Furthermore, lower scattering of rolling angles for sample 4 is evidence of more uniform wettability of this sample.

RESULTS AND DISCUSSION

Laboratory Experiments. The morphology of lasertreated sample 3 is shown in Figure 2 at different magnifications. A detailed study showed that chemisorption of fluorooxysilane did not affect the morphology at the studied 24212


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Figure 4. Time dependences for the fraction of unfrozen droplets at −10 °C (a) and at −18 °C (b) on different substrates: sample 1 (blue diamonds), sample 2 (red triangles), sample 3 (brown stars), sample 4 (green circles). Light brown squares (curve 5) in (b) show the corresponding dependence for the ensemble of brine droplets on the surface of sample 4. The solid lines in (b) are the linear fits to the long-term part of nucleation statistics.

Figure 5. Diagram showing the role of a hydrophobic agent layer in preventing detachment of surface texture elements and thus the formation of additional centers of heterogeneous nucleation.

for droplets on hydrophobic nontreated silicone rubber for all studied temperatures. There are some other examples in the literature of better anti-icing performance of hydrophobic surfaces compared to superhydrophobic ones. For example, longer freezing delay times were reported for sessile droplets8 and impacting droplets.10 Recent studies22,29−31 have shown that weak mechanical stability of the surface texture may cause an essential degeneration of anti-icing properties of superhydrophobic surfaces. The laser-textured surface in our study can be considered as an example of a superhydrophobic surface with a weak mechanical stability of texture. This is due to the fact that during laser ablation nanoparticles containing fragments of PDMS and silica were deposited onto the textured surface from the laser jet. These nanoparticles bind to the surface by weak van der Waals forces, which can be easily destroyed by capillary forces. The siloxane bonds in the PDMS matrix are also prone to hydrolysis in contact with aqueous media. These two factors cause the detachment of nanoparticles from the textured surface on contact with supercooled water, their dispersion in the droplet volume, and transfer to the droplet/air interface. Thus, the mechanical fragility of surface texture causes the appearance of additional centers of heterogeneous nucleation (Figure 5) and, according to nucleation theory,29 results in acceleration of crystallization of the sessile droplet ensemble. This explanation of smaller freezing delay times for droplets deposited onto laser-textured sample 3 than for droplets on hydrophobic nontreated silicone rubber is in good agreement with the increase in freezing delay time for droplets on the laser-textured surface after preliminary cleaning of the surface of sample 3 with an intensive water jet. It is interesting to note

To analyze the superhydrophobic properties of the sample 4 in more detail, we studied the evolution of contact angle and water surface tension on long-term contact of the deposited droplet with the surface of sample 4 in conditions of saturated water vapor. The data obtained at room temperature T = 25 °C are shown in Figure 3 and indicate stability of a very high value of contact angle, whereas the surface tension rapidly decreases to σLV ≈ 61 mN/m. The observed phenomenon of a decrease in water surface tension is characteristic of siloxane rubber and is induced by osmotic effects, which cause a release of monomers or/and oligomers from the PDMS matrix11,12 and their transfer to the droplet/air interface. The initial increase in contact angle is related to the decrease in droplet surface tension, while the preservation of very high value of contact angle for more than 20 h is evidence of high chemical and mechanical stability of the textured layer. As mentioned above, the anti-icing ability of three samples was tested by different methods. The first test is related to a comparison of the crystallization statistics for ensembles of sessile water droplets for different samples. In Figure 4, we show the crystallization statistics for samples at temperatures T = −10 °C and T = −18 °C as a plot of log(N/N0) versus freezing delay time, t, where N is the number of droplets remaining in the supercooled liquid state at time t and N0 is the total number of droplets in the ensemble being studied. The presented data indicate that sessile water droplets may remain in a metastable supercooled state at negative temperatures for a long time on both hydrophobic and superhydrophobic samples. However, the freezing delay time for droplets deposited onto superhydrophobic sample 3 is less than 24213


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freezing delay times for the droplets deposited onto sample 4 (curves 4) essentially exceeded those characteristic of both the hydrophobic nontreated silicone rubber and the laser-treated silicone rubber. Analysis of the freezing delay times obtained for water droplets on sample 4 indicates that a quarter of the whole ensemble of droplets remains unfrozen after reaching the test temperature for as much as 140 h at −10 °C and 20 h at −18 °C. Comparison of curves 1 and 2 obtained for initial silicone rubber without any treatment (sample 1) and the same rubber after deposition of the fluorooxysilane layer (sample 2) shows only minor alteration in freezing kinetics. Here we should mention that both samples 1 and 2 are mechanically stable in contact with the aqueous phase. The main difference between these samples is related to decreased surface energy and slightly increased contact angle for sample 2 in comparison with sample 1. Thus, we may conclude that the significant difference in freezing kinetics of samples 3 and 4 should be mainly attributed to reinforcement of the surface texture for sample 4 due to the binding effect of fluorooxysilane. In order to characterize the anti-icing properties of sample 4 with respect to salt solutions with concentrations characteristic of seawater, we studied the crystallization statistics for ensembles of sessile droplets of a 0.5 M NaCl aqueous solution under the same temperature/humidity conditions. The data presented in Figure 4b allow us to conclude that nucleation kinetics for both deionized water droplets (curve 4) and solution droplets (curve 5) deposited onto sample 4 show linear dependences, log(N/N0) = kt, on long-term exposure of the droplets to supercooling conditions. These linear dependences for the long-term nucleation statistics for both curves 4 and 5 are evidence that a stationary stage of the nucleation process has been established. Freezing kinetics are determined by cumulative rates of homogeneous and heterogeneous nucleation,29 which can be quantified by the effective slope, k, of the linear dependence in the coordinates of Figure 4. The values of the constants k for the stationary stage of ice nucleation were found to be 2 × 10−5 s−1 for water droplets and 1.2 × 10−5 s−1 for brine droplets, indicating nucleation kinetics nearly twice as slow for the latter. Thus, 25% of the whole ensemble of brine droplets retained the liquid state after 40 h of exposure at T = 18 °C. A similar increase in robustness of the metastable liquid state for droplets of a salt solution was shown earlier in papers,32,33 where increased freezing delay time for brine droplets32 and better resistance of the superhydrophobic surfaces to impact icing with saltwater than with pure water33 were detected. In our study,32 we explained the observed increase in the freezing delay time based on the structure of the double electric layer in the vicinity of the hydrophobic surface and the solution/air interface and on the concept of structure making/breaking ions. Thus, the overall experiments and analysis of freezing kinetics on different samples show the indisputable advantages of superhydrophobic sample 4, which has better mechanical stability and water repelling properties. It was interesting to compare the deterioration of water contact angle for samples 3 and 4 upon the temperature decrease to low negative values. Such dependences (Figure 1S in the Supporting Information) obtained for saturated water vapor conditions indicate more significant deterioration of the contact angle for sample 3 in comparison to sample 4 upon temperature decrease with a corresponding increase in time of contact between the droplet and the substrate. Thus, at T = −15.5° the contact angle for sample 3 decreased to 130°, while sample 4 had the value of contact angle of 140°.

Figure 6. (a) Variation of contact angle (triangles), surface tension (circles), and contact diameter (diamonds) of a water droplet on prolonged contact with the superhydrophobic surface of sample 4, under conditions of nearly saturated vapor pressure and a steady decrease in temperature (solid line). (b) Practical work of adhesion of a water droplet to a superhydrophobic surface at different temperatures (diamonds); the dashed line shows the value of W for a water droplet on silicone rubber without treatment at T = 24 °C.

Table 2. Tensile Adhesion Strength with Respect to Glaze Ice at T = −10 °C sample sample 3, laser-treated silicone rubber sample 4, silicone rubber with texture reinforcement by fluorooxysilane sample 4, exposed to outdoor testing for 7 months in winter season

adhesion strength, kPa 399 ± 200 70 ± 17 64 ± 28

that intensive water jet cleaning of samples 1, 2, and 4 had almost no effect on the freezing statistics for droplets deposited onto the corresponding substrates. Deposition of a fluorooxysilane layer on the surface of sample 4 in this study was aimed at improving the mechanical stability of the laser-textured surface and enhancing the robustness of the supercooled state of water droplets on the superhydrophobic surface against freezing. As follows from the data shown in Figure 4, the 24214


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Figure 7. Behavior of samples 1 (hydrophobic silicone rubber) and 4 (superhydrophobic silicone rubber) during rain (a) and during snow (b). Case (a) corresponds to T = +2 °C, wind velocity 1 m/s, and RH (relative humidity) = 90%. In case (b), T = −1 °C, wind velocity 2 m/s, RH = 90%. More details can be found in the Supporting Information Video 1 (rain) and Video 2 (snow).

significantly lower than for sample 1 (dashed line in Figure 6b). This smaller work of adhesion essentially facilitates spontaneous removal of water droplets from the surface at low temperatures under wind or vibrational loads, resulting in better anti-icing performance. To assess the adhesion strength of a glaze ice to the samples 3 and 4 we applied the method of measurement of the tensile adhesion strength similar to that described in ref 37. The test was conducted in a climatic chamber at −10 °C after equilibration of the glaze ice column for 4 h. The experiments were repeated 6 times for the same place of each sample to discover, if any, the degradation of the superhydrophobic state of samples upon ice detachment. It is worth noting that no systematic increase in adhesion strength upon repetitive measurements of ice adhesion strength for the same sample location was observed. To estimate the real stability of sample 4 in the conditions of periodic ice detachment and shedding of supercooled water precipitations we have measured the tensile adhesion strength for the sample preliminarily exposed to outdoor testing for 7 months as described below (see section “Outdoor Experiments”). For this sample, 14 measurements were performed for various locations on the sample surface. The results of measurement of adhesion strength are shown in Table 2. Outdoor Experiments. To estimate the potential of the developed coating for reducing ice and snow accumulation, we performed a cycle of winter environmental tests from October 2016 to April 2017. Two samples were compared in the test: superhydrophobic sample 4, characterized by the reinforced surface texture of silicone rubber, and reference sample 1 of untreated silicone rubber. We monitored the behavior of the samples 24 h a day using a video recording system. The winter season typical of central Russia is characterized by periodic snowfalls, sleet, freezing rain, and freezing drizzle, accompanied by frequent, sometimes sharp temperature changes, as well as high air humidity. The cumulative statistics for temperature and precipitations of the 2016−2017 winter season are shown in Table S1 in the Supporting Information. In such exposure conditions, the surface of sample 1 becomes more rough and contaminated with dust particles, leading to an increase in contact angle in dry conditions (see Table 1). At the same time at long-term contact with the water phase the hydrophobic properties of silicone rubber without additional treatment (sample 1) gradually deteriorate. The decrease in water contact angle due to this degradation results in deposition of a large number of sessile droplets on the sample surface during rain (Figure 7a, left sample) and a large amount of accumulated

To further characterize the anti-icing ability of sample 4, we analyzed the temperature evolution of the practical work of adhesion, W, of supercooled water droplets to the surface of the tested sample. Calculations of W at different temperatures based on the Young−Dupre equation require the measurement of surface tension and receding contact angle for a droplet at various negative temperatures. Since we were interested in the practical work of adhesion at low negative temperatures in conditions similar to typical winter outdoor conditions, the experiments were performed in a Binder MK53 environmental chamber for the temperature range from +23 down to 17 °C. As discussed in the literature,34−36 the correct evaluation of practical work of adhesion is based on the receding contact angle. Therefore, the experiment was organized such that the vapor phase was kept weakly undersaturated. This ensures slow evaporation of a sessile droplet without a noticeable increase in surface tension. Such water droplet evaporation should lead to a decrease in droplet contact diameter. However, concurrent processes, such as the decrease in contact angle induced, on the one hand, by lowering the temperature and, on the other hand, by hydrophilization of the siloxane rubber surface associated with gradual hydrolysis of siloxane bonds on contact with water, eventually result in an increase in droplet contact diameter (Figure 6a). The nonmonotonic behavior of the contact diameter in time and with decreasing temperature is shown in Figure 6a. The time intervals in which the droplet contact diameter shrinks make it possible to determine the receding contact angle θrec for the water droplet. It is worth noting that droplet contact angle gradually deteriorates with an increase in contact time between the droplet and textured silicone rubber and a simultaneous temperature decrease, dropping to a value of 140° at T = −17 °C. The nonmonotonic behavior of droplet surface tension, σLV, is defined by the competition between an increase caused by temperature decrease and a decrease associated with the transition of monomers and oligomers from the siloxane rubber to the droplet surface. Using the Young−Dupre equation, W = σLV (1 + cos θrec), we have calculated the temperature variation of practical work of adhesion (Figure 6b) on the basis of experimental values of surface tension and contact angles (Figure 6a) measured at the corresponding temperatures. For reference, the dashed horizontal line parallel to the abscissa shows the value of W for adhesion of a water droplet to silicone rubber without treatment at T = 24 °C. A comparison of W values leads us to conclude that although the work of adhesion for the superhydrophobic sample noticeably increases at low temperatures (diamonds in Figure 6b) it is still 24215


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Figure 8. Droplet impact dynamics with instant positions of the water droplet (marked by red circles) during dripping at T = −14 °C, zero wind velocity, RH = 80% (a) and at T = −17 °C, zero wind velocity, RH = 75% (b), for 20 μL droplets having T = 0 °C. More details can be found in Video 4 in the Supporting Information.

sample but not onto the superhydrophobic face. It was found that no snow accumulation takes place during heavy snowfall if the wind velocity exceeds 3−5 m/s. Video 2 given in the Supporting Information shows snow sliding along the superhydrophobic sample 4 for 6 h at T = −1 °C without adhesion. In the absence of wind or at lower wind velocity, snowfall results in buildup of a snow cup, which is spontaneously removed from superhydrophobic sample 4 when the thickness of the snow layer exceeds some critical value (see Supporting Information, Video 3). It was also interesting to study the adhesion of water droplets and droplet impact dynamics during dripping onto sample 4 at

snow (Figure 7b, left sample) or slush during sleet. In the latter case, a sharp temperature drop below 0 °C leads to the formation of a patchy (Figure 8a, left sample) or thick ice layer (Figure 8b, left sample). This snow or ice layer typically remains on sample 1 until the temperature rises above zero. Very different behaviors were detected in the same weather conditions for a superhydrophobic sample of type 4. The surface of sample 4 remains dry even during very prolonged rain (Figure 7a, right sample). Video 1 (Supporting Information) shows the behavior of the superhydrophobic sample during 5 h of rain at T = +2 °C. The water droplets were deposited onto the nontreated hydrophobic edges of the 24216


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although the work of adhesion increased with a decrease in contact angles at low negative temperatures it still remained significantly lower than for the nontreated sample. The outdoor experiments showed high resistance of mechanically stable superhydrophobic samples to snow and ice accumulation at temperatures both slightly and significantly lower than T = 0 °C, which well correlates with low values of tensile adhesion strength of ice obtained in laboratory experiments. Finally, the experiments with bouncing droplets show clear evidence of good resistance of our sample to icing at temperatures equal to or higher than T = −17°. At lower temperatures, the effectiveness of droplet rebound decreases. Nevertheless, the total amount of accumulated ice is much less than for the nontreated sample. Thus, based on the set of mutually complementary experimental data presented here, superhydrophobic silicone rubber surfaces with reinforced texture may be considered a very prospective material for outdoor applications in hazardous weather conditions.

low negative temperatures. Frames indicating instant water droplet position during dripping are shown in Figure 8. These experiments were performed at air and sample temperatures T = −14 °C (Figure 8a) and T = −17 °C (Figure 8b), for 20 μL droplets of distilled water having a temperature of 0 °C. The slope of the sample to the horizontal was 25°. In all these experiments, the stage of droplet spreading on impact was followed by complete retraction and bouncing of the droplet (see also Video 4 in the Supporting Information). Here we would like to note that these experiments were performed with a sample exposed for several months to outdoor conditions with periodic atmospheric precipitation and a large amount of atmospheric pollutants (dust, acid/salty rain, etc.), characteristic of a large industrial city (Moscow). Nevertheless, our experiments with dripping droplets show that at T = −17 °C 100% of the droplets rebound from the surface, and even at T = −20 °C, bouncing effectiveness changes slightly to nearly 70%. Similar behavior has been obtained in laboratory experiments by other research groups working with different types of superhydrophobic surfaces.33,38,39 The physical reasons for the decrease in bouncing effectiveness were associated in the literature with an essential increase in the contact area and droplet spreading recoiling time at lower temperatures. However, a new mechanism of heterogeneous nucleation phenomena for impacting droplets was recently suggested.40 According to this mechanism, air bubbles generated during drop impact serve as additional nucleation sites and therefore increase the nucleation rate. The lifetime of these bubbles is very short (tens of milliseconds) at room temperature but increases exponentially as temperature decreases, due to an essential decrease in the diffusion coefficient for gas in water, thus leading to a sharp decrease in bouncing effectiveness of superhydrophobic surfaces at low negative temperatures. It is also interesting that further exposure of sample 4 with frozen deposited (nonbounded) droplets at low negative temperatures was accompanied by ice sublimation much more intensive than that observed for the droplets crystallized on the surface of sample 1. The described behavior is well reproducible in time for different weather impacts, indicating the absence or very weak degradation of the ice- and snowphobic properties of the developed superhydrophobic coating for silicone rubber.

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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.7b05549. Video illustrating the comparative behavior of samples of initial (untreated) PDMS rubber and a rubber with the superhydrophobic coating, under conditions of rain (AVI) Video illustrating the comparative behavior of samples of initial (untreated) PDMS rubber and a rubber with the superhydrophobic coating, under conditions of snow (AVI) Video illustrating the comparative behavior of samples of initial (untreated) PDMS rubber and a rubber with the superhydrophobic coating, under conditions of snow sliding from the superhydrophobic surface under the weak wind (AVI) Video of an experiment with the dripping of water with zero temperature on the rubber surface with a superhydrophobic coating having a temperature of −14 °C (AVI) Table S1 with the data for temperature and precipitations of the 2016−2017 winter season in Moscow, where the outdoor testing of the superhydrophobic sample 4 was performed and compared with behavior of the reference sample 1; Figure S1, showing variation of contact angle for water droplets deposited on samples 3 and 4 with decrease in temperature; Table S2 with the data on wettability characteristics of the sample 4 before and after 25 icing/deicing cycles; and description of results of tape peeling tests for samples 3 and 4 (PDF)

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CONCLUSIONS In this paper, we showed that icephobicity of superhydrophobic materials essentially depends on the mechanical stability of the textured layer. Two types of superhydrophobic silicone rubber samples fabricated in this study, with the same morphology, slightly different surface energy, but significantly different mechanical stability of the textured elements, show very dissimilar freezing kinetics. Our experimental data on freezing delays for sessile aqueous droplets indicate that reinforcement of the surface texture of the superhydrophobic sample due to exploiting the binding effect of a fluorooxysilane layer results in distinctive deceleration of freezing kinetics. Enhancement of the anti-icing ability of our sample is further evidenced by the freezing kinetics for brine droplets. It was found that 25% of the whole ensemble of droplets of 0.5 M NaCl solution retained the liquid state after 40 h of exposure at T = −18 °C. In addition, the measured values of practical work of adhesion for supercooled water droplets on the superhydrophobic sample with strengthened texture showed that

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ludmila B. Boinovich: 0000-0002-1423-695X Notes

The authors declare no competing financial interest. 24217


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ACKNOWLEDGMENTS The work was supported by the Russian Science Foundation (grant # 14-13-01076). Part of experimental studies was performed using the facilities of Center for collective use of scientific equipment CKP FMI IPCE RAS.

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ACS Applied Materials & Interfaces Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2017, 95, 022805.

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Reinforced Superhydrophobic Coating on Silicone Rubber for

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