Article pubs.acs.org/Langmuir
Role of Water Vapor Desublimation in the Adhesion of an Iced Droplet to a Superhydrophobic Surface Ludmila Boinovich* and Alexandre M. Emelyanenko A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Prospect 31, 119071 Moscow, Russia ABSTRACT: The study of the adhesion of solid and liquid aqueous phases to superhydrophobic surfaces has become an attractive topic for researchers in various fields as a vital step in the design of icephobic coatings. The analysis of the available results shows that the experimentally measured values of adhesion strength for superhydrophobic substrates, which in some cases are quite small, are still essentially higher than might be expected from the portion of the actual wetted area. In this study we have considered the peculiarities of the three-phase contact zone between sessile supercooled water or ice droplets and a superhydrophobic coating at negative temperatures (below 0 °C) and during the water−ice phase transition. Two types of superhydrophobic coatings with very different textures were used to analyze the evolution of shape parameters of a sessile water droplet during droplet cooling and freezing. It was shown that the evolution of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface does not undergo essential changes when the droplet is cooled simultaneously with the substrate and the surrounding environment, and the humidity is maintained close to 100% during the cooling process. However, the phase transition from supercooled water to ice droplets leads to the growth of a metastable iced meniscus and a frost halo in the vicinity of the three-phase contact zone. The meniscus effectively increases the area of adhesive contact between the droplet and the substrate. This phenomenon is intrinsically related to the release of the heat of crystallization and is responsible for the enhancement of adhesion to a superhydrophobic substrate upon droplet transition from supercooled water to ice. At the same time, it was shown that the metastable state of the above meniscus leads to its spontaneous sublimation during exposure at negative temperatures.
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separates ice and a vapor phase.10 On a wetted area of a coating, corresponding to direct contact between the ice and the solid surface, the surface nanorelief with heights exceeding the water layer thickness creates an energy barrier against ice sliding relative to the coating along the water layer. Thus, by decreasing the fraction of the wetted surface for the heterogeneous wetting regime of a superhydrophobic coating it is possible to reduce the value of the experimentally measured adhesive strength of the coating/ice contact considerably.3,13 The second mechanism is related to the low adhesive strength of the ice/superhydrophobic coating contact. Breakage of a mechanical contact between two solids (as applied to breakage of the ice/substrate contact) practically cannot occur by an isothermal reversible process. Contact breakage along an interface is characterized by an adhesive strength that equals the force required to break the adhesive bonding per the apparent adhesive contact area. Here, the adhesive strength is determined to a large extent by the number of defects in the near-surface layer and the character of deformation in the contacting bodies.14 For a long time, hydrophobic coatings with low surface energy that did not form covalent chemical bonds with the
INTRODUCTION The study of the adhesion of solid and liquid aqueous phases to superhydrophobic surfaces has become an attractive topic for researchers in various fields as a vital step in the design of icephobic coatings.1−13 As was shown in numerous studies, superhydrophobic coatings on metal surfaces demonstrate essentially lower adhesion to both liquid water and ice than bare metals or metals covered with hydrophobic coatings.1,2,4,7−13 As was discussed in one of our earlier studies,10 two mechanisms may be responsible for the low adhesion strength of contact of the crystalline aqueous phase to a superhydrophobic substrate. The first mechanism is related to ice premelting or the existence of a water layer at negative temperatures, either on the ice surface or between the ice and a substrate. The shift of the water triple point at the ice/vapor and ice/solid interfaces toward low (negative) temperatures is considered to be the main physical reason for the existence of such a layer. A polymolecular water layer at the ice/vapor phase and ice/ hydrophobic surface interfaces acts as a lubricating layer of a liquid antiadhesive agent3,7,13 and thus makes it possible to considerably decrease the ice−coating adhesion strength under shear load and combined shear conditions. However, the above phenomenon is generally observed only at those points of heterogeneous ice−coating contact where a water layer © XXXX American Chemical Society
Received: August 28, 2014 Revised: October 2, 2014
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Figure 1. Surface morphologies of the superhydrophobic coatings on anodized (a−c) and laser-treated aluminum (d−f). Scale bars are (a) 2 μm, (b, c, f) 100 nm, (d) 10 μm, (e) and 300 nm. The coating on anodized aluminum is characterized by highly ordered porous texture with parallel pores having a diameter of ∼100 nm (a, b), ending with single fibers (some fibers are indicated by arrows in (c). Multimodal texture having regular surface ripples with a period of a few tens of micrometers (d) decorated with nearly spherical aggregates (e) constituted of nanoparticles (f) was characteristic of the coating obtained on laser-treated aluminum.
some cases,17−19 the measured values of the adhesion strength of ice to a highly hydrophobic coating even exceed those on smooth hydrophobic substrates. Such enhanced adhesion strength of ice to highly hydrophobic coatings was associated in the literature with the growth of the droplet/coating contact area resulting from both contact angle deterioration at negative temperatures14,20,21 and phase transition.17 However, the physical nature of above phenomenon still requires a deeper understanding. Our aim in this article is to consider the peculiarities of the three-phase contact zone between sessile supercooled water/ice droplets and a superhydrophobic coating at negative temperatures and during the water−ice phase transition. It will be shown that the evolution of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface does not undergo essential changes when the droplet is cooled simultaneously with the substrate and the surrounding environment whereas the humidity is maintained at close to 100% during the cooling process. However, the phase transition from supercooled water to ice droplets leads to the growth of a metastable iced meniscus and frost halo in the vicinity of the three-phase contact zone. This phenomenon is intrinsically related to the release of the heat of crystallization and is responsible for the enhancement of adhesion to a superhydrophobic substrate upon droplet transition from supercooled water to ice. At the same time, it will be shown that the metastable state of the above meniscus leads to its spontaneous sublimation during exposure at negative temperatures. It should be noted that the formation of a frost halo around the freezing droplet was observed in a recent study22 of water droplets freezing on hydrophilic substrates (water contact angle of around 72°) with different thermal conductivities. However, the results obtained in our study for superhydrophobic substrates under saturated vapor conditions show a significantly different character of water vapor desublimation around the droplet during the freezing process.
components of an aqueous medium were used to decrease the adhesive strength of ice contact with engineering materials. Recent studies have shown that the adhesion strength of ice to superhydrophobic coatings can be considerably lower than that to hydrophobic coatings.1,4,6,13,15,16 However, a correlation between an increase in the effective contact angle and a decrease in the adhesive strength was observed6,17 only for true superhydrophobic coatings, i.e., coatings with low hysteresis and an advancing contact angle larger than 150°. It should be recalled here that if water droplet crystallization on top of a superhydrophobic surface is not accompanied by the transition from the heterogeneous to homogeneous regime, then the ice partially contacts the coating surface and partially contacts the air bubbles trapped in the grooves of the relief. In this case, the actual adhesive strength is associated with the breakage of intermolecular interactions between the ice and the coating only along the surface of the real contact with the coating; hence, it is determined by the chemical structure of the coating and the actual adhesive contact area. For true superhydrophobic surfaces, the latter is defined by the product of the wetted area and the roughness factor of the wetted area. Superhydrophobic surfaces are generally characterized by a small fraction of wetted area f and a moderate wetted surface roughness determined by the shape of the texture elements. Thus, because of the small actual contact area, the measured adhesive strength of contact between ice and a superhydrophobic surface is much smaller than in the case of hydrophobic substrates with a wetted area fraction of f = 1, whereas the actual adhesive strengths per unit of contact area may be similar in these cases. Although these issues have been discussed in the literature, the analysis of the available results shows that the experimentally measured values of adhesion strength for superhydrophobic substrates, which in some cases are quite small,8,10,13,15 are still essentially higher than might be expected from the portion of the actual wetted area. Furthermore, in B
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equilibrated, manipulation with an angular positioner allowed us to change the sample surface tilt in a controllable manner and detect the rolling angle by averaging over 10 different droplets on the same substrate. Typical contact angles and rolling angles for both types of superhydrophobic coatings are shown in Table 1.
MATERIALS AND METHODS
In this article, we studied the temperature evolution of parameters of a 15 μL water droplet deposited on an aluminum substrate with a superhydrophobic coating. In our experiments, droplet parameters such as the contact angle, base diameter, apex radius of the water droplet, droplet volume, droplet surface area, and liquid surface tension to density ratio were monitored as the temperature decreased. The rate of droplet cooling from room temperature to freezing temperature was 1°/h. In the course of cooling, the droplet was exposed to saturated vapor conditions at positive temperatures and to nearly saturated vapor conditions at negative temperatures, as described earlier.23 Deionized water (electrical resistance 15 MΩ m) was used as the testing liquid for the experiment. To control the temperature and avoid vibrational perturbations of the freezing process, the cell was placed on an antivibration support inside a Binder MK53 environmental chamber. Because it has been mentioned many times in the literature1,2,10,17 that the roughness and shape of texture elements influence the deterioration of the contact angle upon cooling and affect the adhesion strength of ice to superhydrophobic coatings, we performed our experiments on superhydrophobic coatings with two very different types of texture. The coatings were prepared on aluminum plates with typical plate sizes of 20 mm × 10 mm × 1 mm obtained by rolling A0 aluminum alloy wires (>99.0 wt % Al). For the fabrication of the first type of texture with multimodal roughness, anodic oxidation of an aluminum sample was performed in the galvanostatic regime in a phosphoric acid solution, as described in ref 23, followed by washing the anodized sample with distilled water, drying in an oven at 130 °C, and finally chemical adsorption of the hydrophobic agent methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]propyl}-silane from solution in dehydrated decane. The prepared coating is characterized by a highly ordered porous texture with parallel pores (pore diameter of ∼100 nm) ending with single fibers (Figure 1a−c). The pore walls demonstrated an ordered, highly organized perforation by nano-openings with a circular cross section of ∼20 nm in diameter. Preparation of the second type of superhydrophobic coating on aluminum included degreasing the aluminum plates, pulsed nanosecond IR laser treatment (ytterbium fiber laser with a wavelength of 1.064 μm, pulse duration of 50 ns, and repetition rate of 20 kHz), washing with distilled water, drying in an oven at 130 °C, and finally chemical adsorption of the same hydrophobic agent, methoxy-{3[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]propyl}-silane, from solution in dehydrated decane. Multimodal texture having regular surface ripples with a period of a few tens of micrometers (Figure 1d) decorated by nearly spherical aggregates (Figure 1e) composed of nanoparticles (Figure 1f) was characteristic of the latter type of coating. Heat treatment of both types of coatings in the oven at 140 °C in the presence of water vapor after solvent evaporation leads to crosslinking between molecules of the hydrophobic agent24 and the formation of a surface monolayer of fluorooxysilane that is chemically stable to long-term contact with the aqueous phase. The morphologies of both types of surfaces are shown in Figure 1. The microstructure of the samples was studied using a Carl Zeiss Supra 40 VP scanning electron microscope. Micrographs were taken at 5 and 10 kV acceleration voltages using secondary (InLens, SE2) electron detectors. Characterization of the wettability of the coatings was based on contact and rolling angle measurements. We used digital video image processing of sessile droplets to analyze the droplet shape parameters. The homemade experimental setup for recording optical images of sessile droplets and the software for the subsequent determination of droplet parameters using the Laplace curve-fitting routine were described earlier.25 In order to characterize the wetting of different coatings, initial contact angles for 10−15 μL droplets were measured on five different surface locations for each sample. For the measurement of the rolling angle, 10 μL droplets were deposited on the surface. After the initial droplet shape was
Table 1. Contact (CA) and Rolling (RA) Angles on Different Samples Used for Experiments with Drops Cooling and Freezing sample 1: superhydrophobic coating on anodized aluminum
sample 2: superhydrophobic coating on laser-treated aluminum
CA, deg
RA, deg
CA, deg
RA, deg
167.5 ± 2.6
7.6 ± 3.3
170.5 ± 1.6
1.3 ± 0.5
The experiment for the analysis of the temperature behavior of a water droplet contact angle and contact diameter was organized as follows. The droplet was deposited on the substrate inside a doublewalled cell. The water vapor saturation inside the cell at temperatures above 0 °C was achieved by pouring a 2 mm water layer on the bottom of the cell, which had a large water evaporation area. After the cell was sealed and the vapor phase was equilibrated in the cell for 3 h, the temperature in the environmental chamber was lowered at a rate of 1°/h.
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RESULTS AND DISCUSSION The two types of superhydrophobic coatings discussed above were used to study the evolution of droplet shape parameters during droplet cooling and freezing. The temperature behavior of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface fabricated by laser treatment (sample 2) is shown in Figure 2a. As follows from the data in Figure 2a, weak deterioration of the contact angle is observed during droplet cooling. The decrease in contact angle in the temperature interval from 20 to −5 °C is accompanied by an increase in contact diameter, indicating that in this temperature interval we are dealing with an advancing contact angle with weak descent defined by the temperature dependence of surface forces.26 The diminution of droplet volume in the first 28 h of the experiment did not exceed 1%. However, at lower temperatures, the decrease in relative vapor pressure above the ice layer on the bottom of the cell with respect to the supercooled water droplet (Figure 2b) resulted in an intensification of droplet evaporation, causing a decrease in the droplet contact diameter. A more rapid decrease in contact angle in the interval from −5 to −12 °C indicates that the transition from advancing to receding contact angle takes place because of droplet evaporation. Further droplet cooling leads to weak deterioration of the receding contact angle until the droplet freezes. The total diminution of droplet volume for the whole cooling time from room temperature until droplet freezing did not exceed 5%. Because nucleation in supercooled water droplets is a stochastic process, the freezing temperature for individual droplets varies over a wide range. The behavior shown in Figure 2a is typical of this type of superhydrophobic coating, and the difference from sample to sample was associated with the final freezing temperature. The freezing event is easily detected by the disappearance of the bright spot of transmitted light on the droplet image upon freezing, followed by the formation of a frozen cap in the apex of the droplet (Figure 3a− e). In our experiments for freezing temperatures in the range of −15 to −21 °C, the receding water contact angle varied from C
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vapor pressure in the vicinity of the droplet, as determined by the actual aqueous phase temperature.31 The local excess in the vapor pressure compared to the saturated values characteristic of the substrate temperature results in the desublimation of supersaturated vapors inside the gap between the droplet and the substrate and also frost deposition on the substrate under the drop. It is interesting that in the case of a superhydrophobic substrate and nearly saturated ambient atmosphere, no water microdroplet condensation was observed, in contrast to the findings for hydrophilic substrates.22 The appearance of an iced meniscus and frost around the droplet similar to that shown in Figure 3g,h was detected for the process of droplet freezing on top of sample 2. Figure 4a shows the details of meniscus formation versus time for the digitized profile of a sessile droplet, and it may be concluded that the desublimation stage is very fast and is typically completed within a time interval on the order of 1 min (most of the meniscus is formed within 20−30 s). Very rapid spatial relaxation of supersaturated vapor pressure to the values determined by the temperature of the frozen water layer on the bottom of the cell is proven by the fact that the size of the frost halo is very limited and is substantially less than the droplet diameter. It is worth noting that in the first stage of freezing (during 1 s), the contact angle discontinuously decreased slightly while the droplet edge moved moderately upward (dotted magenta lines) and remained essentially unchanged over the next several hours of exposure of the frozen droplet at constant negative temperature and saturated vapors. This indicates that, on the one hand, during freezing, the capillary condensation initiated by short-term droplet heating causes an increase in droplet contact diameter associated with the growth of the meniscus. On the other hand, vapor desublimation between the asperities of the superhydrophobic coating leads to the transition from the heterogeneous to the mixed heterogeneous/homogeneous regime, which is illustrated by an abrupt but not very essential shift in the iced droplet edge. The increase in the ice/substrate contact area associated with the formation of an iced meniscus and frost halo under the droplet can be considered to be the main physical reasons for the enhancement of the actual adhesion strength of the frozen droplet to the superhydrophobic coating. At the same time, it should be stressed that neither the meniscus nor the frost halo corresponds to the stable state of the system represented by the iced droplet on top of the superhydrophobic coating. The spontaneous evolution of the frozen meniscus and the frost halo at the droplet edge with time of exposure to nearly saturated vapors, as can be seen in Figure 4b, indicates the sublimation of water from both the iced meniscus and the frost halo and the tending of the droplet contact diameter to the value characteristic of the droplet just before freezing. This is a long-range process that proceeds over many hours. However, it will favor a decrease in the actual adhesion strength of the iced droplet and will facilitate the detachment of the droplet under an external load.
Figure 2. (a) Variation of the contact diameter (blue circles) and the contact angle (red triangles) of a water droplet deposited on sample 2 with a superhydrophobic surface fabricated by laser treatment as a function of temperature during droplet cooling from 20 °C until droplet freezing at −17.3 °C. (b) At positive temperatures, the water layer on the bottom of the experimental cell supports the 100% relative humidity in the cell; however, at negative temperatures, after the freezing of this layer, the vapors in the cell are undersaturated with respect to supercooled liquid water in the droplet. The graph shows the temperature dependence of relative undersaturation, that is, the ratio of equilibrium vapor pressure above ice, Pice, to that above supercooled water, Pscw. The green line was calculated from empirical equations for vapor pressures,27 and points were calculated from experimental data.28
163 to 161°, whereas the contact diameter was nearly the same as for the freshly deposited droplet at 20 °C. The behavior of the contact angle and contact diameter for the superhydrophobic coating obtained by anodic oxidation23 is similar to those described above for the laser-treated aluminum sample, but with a receding angle slightly greater than 168° just before droplet crystallization. We did not obtain contact angle jumps at negative temperatures in a nearly saturated vapor environment on either of the superhydrophobic substrates. (Such jumps, if observed, could be considered to be an indication of capillary condensation between the asperities and a transition from the heterogeneous to homogeneous or mixed heterogeneous/homogeneous wetting regime.) However, the situation is very different during droplet crystallization. Figure 3a−e shows the optical images of a freezing water droplet on top of sample 1 at different times during the crystallization process. The evolution of the peculiarities of droplet shape can be analyzed from the digitized droplet images shown in Figure 3f−h. It follows that droplet crystallization is accompanied by a variation in droplet shape as a whole, the simultaneous growth of an iced meniscus between the droplet and substrate, and frost halo formation in the vicinity of the three-phase contact zone. The appearance of this meniscus and halo is easily understood if one takes into account that the first stage of droplet freezing can be considered to be quasiadiabatic. That is why the release of the heat of freezing results in some increase in the droplet temperature compared to the temperature of the ambient air and substrate. This phenomenon was detected experimentally22,29,30 and leads to a short-term local increase in
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CONCLUSIONS Two types of superhydrophobic coatings with very different textures were used in this study to analyze the evolution of shape parameters of a sessile water droplet during droplet cooling and freezing. The behavior of the contact angle and contact diameter of the water droplet is similar for both coatings in a saturated water vapor atmosphere. Weak descent D
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Figure 3. Images of a droplet on the surface of sample 1 (superhydrophobic coating on anodized aluminum): (a) liquid droplet just before freezing and (b) 5 s, (c) 15 s, (d) 20 s, and (e) 1 min after the start of freezing. Digitized profiles of droplets at different times (f) and details of menisci at the left (g) and right (h) droplet edges. It is easily seen that the maximum extension of the meniscus is achieved at about 30 s, and after that the meniscus recedes because of sublimation. The orange dashed line in (f−h) indicates the theoretical Laplacian fit to the liquid droplet profile and the position of the substrate.
Figure 4. Evolution of the droplet edge profile and of the iced meniscus and frost halo at the droplet edge upon freezing (a) and with time of exposure to nearly saturated vapors (b). Black solid lines indicate the digitized profile of a liquid droplet just before freezing. Upon freezing, the contact angle discontinuously decreased slightly whereas the droplet edge moved moderately upward (dotted magenta lines) and remained essentially unchanged over the next several hours of exposure at constant negative temperature and nearly saturated vapors. The iced meniscus formed intensively within first minute after the start of droplet freezing, as can be seen in panel (a). Further exposure of the frozen droplet in nearly saturated vapor pressure relative to the iced droplet led to the sublimation of ice from the meniscus and the frost halo from the substrate, as shown on panel (b).
of the contact angle at positive and negative temperatures is a result of the temperature dependence of surface forces. Cooling the cell to below zero leads to the crystallization of the water layer on the bottom of the cell, which was used as a source of saturated vapor inside the cell. The decrease in the relative
vapor humidity above the ice with respect to the supercooled water droplet initiated by this crystallization intensifies droplet evaporation, which is followed by the transition from advancing to receding contact angle. The typical very high values of receding angles detected on both substrates allowed us to E
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(11) Sarshar, M. A.; Swarctz, C.; Hunter, S.; Simpson, J.; Choi, C. H. Effects of Contact Angle Hysteresis on Ice Adhesion and Growth on Superhydrophobic Surfaces under Dynamic Flow Conditions. Colloid Polym. Sci. 2013, 291, 427−435. (12) Arianpour, F.; Farzaneh, M.; Kulinich, S. A. Hydrophobic and Ice-Retarding Properties of Doped Silicone Rubber Coatings. Appl. Surf. Sci. 2013, 265, 546−552. (13) Boinovich, L. B.; Zhevnenko, S. N.; Emelyanenko, A. M.; Goldstein, R. V.; Epifanov, V. P. Adhesive Strength of the Contact of Ice with a Superhydrophobic Coating. Dokl. Chem. 2013, 448, 71−75. (14) Nosonovsky, M.; Hejazi, V. Why Superhydrophobic Surfaces Are Not Always Icephobic. ACS Nano 2012, 6, 8488−8491. (15) Dodiuk, H.; Kenig, S.; Dotan, A. Do Self-Cleaning Surfaces Repel Ice? J. Adhes. Sci. Technol. 2012, 26, 701−714. (16) Meuler, A. J.; Smith, J. D.; Varanasi, K. K.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Relationships between Water Wettability and Ice Adhesion. ACS Appl. Mater. Interfaces 2010, 2, 3100−3110. (17) Kulinich, S. A.; Farzaneh, M. How Wetting Hysteresis Influences Ice Adhesion Strength on Superhydrophobic Surfaces. Langmuir 2009, 25, 8854−8856. (18) Yang, S. Q.; Xia, Q. A.; Zhu, L.; Xue, J. A.; Wang, Q. J.; Chen, Q. M. Research on the Icephobic Properties of Fluoropolymer-Based Materials. Appl. Surf. Sci. 2011, 257, 4956−4962. (19) Zou, M.; Beckford, S.; Wei, R.; Ellis, C.; Hatton, G.; Miller, M. A. Effects of Surface Roughness and Energy on Ice Adhesion Strength. Appl. Surf. Sci. 2011, 257, 3786−3792. (20) Karmouch, R.; Ross, G. G. Experimental Study on the Evolution of Contact Angles with Temperature near the Freezing Point. J. Phys. Chem. C 2010, 114, 4063−4066. (21) He, M.; Li, H.; Wang, J.; Song, Y. Superhydrophobic Surface at Low Surface Temperature. Appl. Phys. Lett. 2011, 98, 093118. (22) Jung, S.; Tiwari, M. K.; Poulikakos, D. Frost Halos from Supercooled Water Droplets. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 16073−16078. (23) Boinovich, L. B.; Emelyanenko, A. M.; Korolev, V. V.; Pashinin, A. S. Effect of Wettability on Sessile Drop Freezing. When Superhydrophobicity Stimulates Extreme Freezing Delay. Langmuir 2014, 30, 1659−1668. (24) Boinovich, L.; Emelyanenko, A. A Wetting Experiment as a Tool to Study the Physicochemical Processes Accompanying the Contact of Hydrophobic and Superhydrophobic Materials with Aqueous Media. Adv. Colloid Interface Sci. 2012, 179, 133−141. (25) Emelyanenko, A. M.; Boinovich, L. B. Application of Dynamic Thresholding of Video Images for Measuring the Interfacial Tension of Liquids and Contact Angles. Instrum. Exp. Tech. 2002, 45, 44−49. (26) Boinovich, L.; Emelyanenko, A. Wetting and Surface Forces. Adv. Colloid Interface Sci. 2011, 165, 60−69. (27) Murphy, D. M.; Koop, T. Review of the Vapour Pressures of Ice and Supercooled Water for Atmospheric Applications. Q. J. R. Meteorol. Soc. 2005, 131, 1539−1565. (28) Spravochnik Khimika (Chemist’s Handbook); Nikolsky, B. P., Ed.; Goskhimizdat: Leningrad, 1962; Vol. 1. (29) Alizadeh, A.; Yamada, M.; Li, R.; Shang, W.; Otta, S.; Zhong, S.; Ge, L.; Dhinojwala, A.; Conway, K. R.; Bahadur, V.; Vinciquerra, A. J.; Stephens, B.; Blohm, M. L. Dynamics of Ice Nucleation on Water Repellent Surfaces. Langmuir 2012, 28, 3180−3186. (30) Yin, L.; Xia, Q.; Xue, J.; Yang, S.; Wang, Q.; Chen, Q. In Situ Investigation of Ice Formation on Surfaces with Representative Wettability. Appl. Surf. Sci. 2010, 256, 6764−6769. (31) Bakanov, S. P. The Effect of Phase and other Transformations at the Surface of Particles on the Thermophoresis of Aerosols. Colloid J. 1995, 57, 731−735.
conclude that at negative temperatures the wetting regime of superhydrophobic substrates remains heterogeneous. The heterogeneity is responsible for the weak adhesion of droplets to a superhydrophobic coating, which was confirmed by the experimental observation of the spontaneous removal of droplets from a horizontal superhydrophobic surface during shaking at negative temperatures. However, upon droplet crystallization, the release of heat of freezing leads to some increase in the actual droplet temperature and a local oversaturation of vapors in the vicinity of the droplet. As a result, the simultaneous growth of an iced meniscus inside the gap between the droplet and substrate and frost deposition in the vicinity of the three-phase contact zone occur within 1 min. These phenomena will be characteristic of superhydrophobic coatings of any nature and follow from the universal phenomenon of the release of the latent heat of phase transition in the crystallizing droplet. However, it may be essentially suppressed if the substrate temperature is kept higher than the droplet and the ambient atmosphere temperature. Another important observation of this study is related to the establishment of a metastable state of the desublimated meniscus and the frost halo around the droplet base. This causes the spontaneous sublimation of water from both the frozen meniscus and the frost halo with time of exposure to saturated vapors. Although this process takes many hours, it will favor a decrease in the actual adhesion strength of the iced droplet and can be successfully used in practice to facilitate the detachment of ice from superhydrophobic surfaces.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was performed with the financial support of the Russian Scientific Foundation (project no. 14-13-01076). REFERENCES
(1) He, Y.; Jiang, C. Y.; Cao, X. B.; Chen, J.; Tian, W.; Yuan, W. Z. Reducing Ice Adhesion by Hierarchical Micro-Nano-Pillars. Appl. Surf. Sci. 2014, 305, 589−595. (2) Davis, A.; Yeong, Y. H.; Steele, A.; Bayer, I. S.; Loth, E. Superhydrophobic Nanocomposite Surface Topography and Ice Adhesion. ACS Appl. Mater. Interfaces 2014, 6, 9272−9279. (3) Dou, R. M.; Chen, J.; Zhang, Y. F.; Wang, X. P.; Cui, D.; Song, Y. L.; Jiang, L.; Wang, J. J. Anti-icing Coating with an Aqueous Lubricating Layer. ACS Appl. Mater. Interfaces 2014, 6, 6998−7003. (4) Dotan, A.; Dodiuk, H.; Laforte, C.; Kenig, S. The Relationship between Water Wetting and Ice Adhesion. J. Adhes. Sci. Technol. 2009, 23, 1907−1915. (5) Farhadi, S.; Farzaneh, M.; Kulinich, S.A. Anti-icing Performance of Superhydrophobic Surfaces. Appl. Surf. Sci. 2011, 257, 6264−6269. (6) Kulinich, S. A.; Farzaneh, M. Ice Adhesion on SuperHydrophobic Surfaces. Appl. Surf. Sci. 2009, 255, 8153−8157. (7) Lv, J. Y.; Song, Y. L.; Jiang, L.; Wang, J. J. Bio-Inspired Strategies for Anti-Icing. ACS Nano 2014, 8, 3152−3169. (8) Zhu, H.; Guo, Z. G.; Liu, W. M. Adhesion Behaviors on Superhydrophobic Surfaces. Chem. Commun. 2014, 50, 3900−3913. (9) Wang, Y. Y.; Xue, J.; Wang, Q. J.; Chen, Q. M.; Ding, J. F. Verification of Icephobic/Anti-Icing Properties of a Superhydrophobic Surface. ACS Appl. Mater. Interfaces 2013, 5, 3370−3381. (10) Boinovich, L. B.; Emelyanenko, A.M. Anti-Icing Potential of Superhydrophobic Coatings. Mendeleev Commun. 2013, 23, 3−10. F
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Role of Water Vapor Desublimation in the Adhesion of an Iced Droplet to a Superhydrophobic Surface Ludmila Boinovich* and Alexandre M. Emelyanenko A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Prospect 31, 119071 Moscow, Russia ABSTRACT: The study of the adhesion of solid and liquid aqueous phases to superhydrophobic surfaces has become an attractive topic for researchers in various fields as a vital step in the design of icephobic coatings. The analysis of the available results shows that the experimentally measured values of adhesion strength for superhydrophobic substrates, which in some cases are quite small, are still essentially higher than might be expected from the portion of the actual wetted area. In this study we have considered the peculiarities of the three-phase contact zone between sessile supercooled water or ice droplets and a superhydrophobic coating at negative temperatures (below 0 °C) and during the water−ice phase transition. Two types of superhydrophobic coatings with very different textures were used to analyze the evolution of shape parameters of a sessile water droplet during droplet cooling and freezing. It was shown that the evolution of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface does not undergo essential changes when the droplet is cooled simultaneously with the substrate and the surrounding environment, and the humidity is maintained close to 100% during the cooling process. However, the phase transition from supercooled water to ice droplets leads to the growth of a metastable iced meniscus and a frost halo in the vicinity of the three-phase contact zone. The meniscus effectively increases the area of adhesive contact between the droplet and the substrate. This phenomenon is intrinsically related to the release of the heat of crystallization and is responsible for the enhancement of adhesion to a superhydrophobic substrate upon droplet transition from supercooled water to ice. At the same time, it was shown that the metastable state of the above meniscus leads to its spontaneous sublimation during exposure at negative temperatures.
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separates ice and a vapor phase.10 On a wetted area of a coating, corresponding to direct contact between the ice and the solid surface, the surface nanorelief with heights exceeding the water layer thickness creates an energy barrier against ice sliding relative to the coating along the water layer. Thus, by decreasing the fraction of the wetted surface for the heterogeneous wetting regime of a superhydrophobic coating it is possible to reduce the value of the experimentally measured adhesive strength of the coating/ice contact considerably.3,13 The second mechanism is related to the low adhesive strength of the ice/superhydrophobic coating contact. Breakage of a mechanical contact between two solids (as applied to breakage of the ice/substrate contact) practically cannot occur by an isothermal reversible process. Contact breakage along an interface is characterized by an adhesive strength that equals the force required to break the adhesive bonding per the apparent adhesive contact area. Here, the adhesive strength is determined to a large extent by the number of defects in the near-surface layer and the character of deformation in the contacting bodies.14 For a long time, hydrophobic coatings with low surface energy that did not form covalent chemical bonds with the
INTRODUCTION The study of the adhesion of solid and liquid aqueous phases to superhydrophobic surfaces has become an attractive topic for researchers in various fields as a vital step in the design of icephobic coatings.1−13 As was shown in numerous studies, superhydrophobic coatings on metal surfaces demonstrate essentially lower adhesion to both liquid water and ice than bare metals or metals covered with hydrophobic coatings.1,2,4,7−13 As was discussed in one of our earlier studies,10 two mechanisms may be responsible for the low adhesion strength of contact of the crystalline aqueous phase to a superhydrophobic substrate. The first mechanism is related to ice premelting or the existence of a water layer at negative temperatures, either on the ice surface or between the ice and a substrate. The shift of the water triple point at the ice/vapor and ice/solid interfaces toward low (negative) temperatures is considered to be the main physical reason for the existence of such a layer. A polymolecular water layer at the ice/vapor phase and ice/ hydrophobic surface interfaces acts as a lubricating layer of a liquid antiadhesive agent3,7,13 and thus makes it possible to considerably decrease the ice−coating adhesion strength under shear load and combined shear conditions. However, the above phenomenon is generally observed only at those points of heterogeneous ice−coating contact where a water layer © XXXX American Chemical Society
Received: August 28, 2014 Revised: October 2, 2014
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Figure 1. Surface morphologies of the superhydrophobic coatings on anodized (a−c) and laser-treated aluminum (d−f). Scale bars are (a) 2 μm, (b, c, f) 100 nm, (d) 10 μm, (e) and 300 nm. The coating on anodized aluminum is characterized by highly ordered porous texture with parallel pores having a diameter of ∼100 nm (a, b), ending with single fibers (some fibers are indicated by arrows in (c). Multimodal texture having regular surface ripples with a period of a few tens of micrometers (d) decorated with nearly spherical aggregates (e) constituted of nanoparticles (f) was characteristic of the coating obtained on laser-treated aluminum.
some cases,17−19 the measured values of the adhesion strength of ice to a highly hydrophobic coating even exceed those on smooth hydrophobic substrates. Such enhanced adhesion strength of ice to highly hydrophobic coatings was associated in the literature with the growth of the droplet/coating contact area resulting from both contact angle deterioration at negative temperatures14,20,21 and phase transition.17 However, the physical nature of above phenomenon still requires a deeper understanding. Our aim in this article is to consider the peculiarities of the three-phase contact zone between sessile supercooled water/ice droplets and a superhydrophobic coating at negative temperatures and during the water−ice phase transition. It will be shown that the evolution of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface does not undergo essential changes when the droplet is cooled simultaneously with the substrate and the surrounding environment whereas the humidity is maintained at close to 100% during the cooling process. However, the phase transition from supercooled water to ice droplets leads to the growth of a metastable iced meniscus and frost halo in the vicinity of the three-phase contact zone. This phenomenon is intrinsically related to the release of the heat of crystallization and is responsible for the enhancement of adhesion to a superhydrophobic substrate upon droplet transition from supercooled water to ice. At the same time, it will be shown that the metastable state of the above meniscus leads to its spontaneous sublimation during exposure at negative temperatures. It should be noted that the formation of a frost halo around the freezing droplet was observed in a recent study22 of water droplets freezing on hydrophilic substrates (water contact angle of around 72°) with different thermal conductivities. However, the results obtained in our study for superhydrophobic substrates under saturated vapor conditions show a significantly different character of water vapor desublimation around the droplet during the freezing process.
components of an aqueous medium were used to decrease the adhesive strength of ice contact with engineering materials. Recent studies have shown that the adhesion strength of ice to superhydrophobic coatings can be considerably lower than that to hydrophobic coatings.1,4,6,13,15,16 However, a correlation between an increase in the effective contact angle and a decrease in the adhesive strength was observed6,17 only for true superhydrophobic coatings, i.e., coatings with low hysteresis and an advancing contact angle larger than 150°. It should be recalled here that if water droplet crystallization on top of a superhydrophobic surface is not accompanied by the transition from the heterogeneous to homogeneous regime, then the ice partially contacts the coating surface and partially contacts the air bubbles trapped in the grooves of the relief. In this case, the actual adhesive strength is associated with the breakage of intermolecular interactions between the ice and the coating only along the surface of the real contact with the coating; hence, it is determined by the chemical structure of the coating and the actual adhesive contact area. For true superhydrophobic surfaces, the latter is defined by the product of the wetted area and the roughness factor of the wetted area. Superhydrophobic surfaces are generally characterized by a small fraction of wetted area f and a moderate wetted surface roughness determined by the shape of the texture elements. Thus, because of the small actual contact area, the measured adhesive strength of contact between ice and a superhydrophobic surface is much smaller than in the case of hydrophobic substrates with a wetted area fraction of f = 1, whereas the actual adhesive strengths per unit of contact area may be similar in these cases. Although these issues have been discussed in the literature, the analysis of the available results shows that the experimentally measured values of adhesion strength for superhydrophobic substrates, which in some cases are quite small,8,10,13,15 are still essentially higher than might be expected from the portion of the actual wetted area. Furthermore, in B
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equilibrated, manipulation with an angular positioner allowed us to change the sample surface tilt in a controllable manner and detect the rolling angle by averaging over 10 different droplets on the same substrate. Typical contact angles and rolling angles for both types of superhydrophobic coatings are shown in Table 1.
MATERIALS AND METHODS
In this article, we studied the temperature evolution of parameters of a 15 μL water droplet deposited on an aluminum substrate with a superhydrophobic coating. In our experiments, droplet parameters such as the contact angle, base diameter, apex radius of the water droplet, droplet volume, droplet surface area, and liquid surface tension to density ratio were monitored as the temperature decreased. The rate of droplet cooling from room temperature to freezing temperature was 1°/h. In the course of cooling, the droplet was exposed to saturated vapor conditions at positive temperatures and to nearly saturated vapor conditions at negative temperatures, as described earlier.23 Deionized water (electrical resistance 15 MΩ m) was used as the testing liquid for the experiment. To control the temperature and avoid vibrational perturbations of the freezing process, the cell was placed on an antivibration support inside a Binder MK53 environmental chamber. Because it has been mentioned many times in the literature1,2,10,17 that the roughness and shape of texture elements influence the deterioration of the contact angle upon cooling and affect the adhesion strength of ice to superhydrophobic coatings, we performed our experiments on superhydrophobic coatings with two very different types of texture. The coatings were prepared on aluminum plates with typical plate sizes of 20 mm × 10 mm × 1 mm obtained by rolling A0 aluminum alloy wires (>99.0 wt % Al). For the fabrication of the first type of texture with multimodal roughness, anodic oxidation of an aluminum sample was performed in the galvanostatic regime in a phosphoric acid solution, as described in ref 23, followed by washing the anodized sample with distilled water, drying in an oven at 130 °C, and finally chemical adsorption of the hydrophobic agent methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]propyl}-silane from solution in dehydrated decane. The prepared coating is characterized by a highly ordered porous texture with parallel pores (pore diameter of ∼100 nm) ending with single fibers (Figure 1a−c). The pore walls demonstrated an ordered, highly organized perforation by nano-openings with a circular cross section of ∼20 nm in diameter. Preparation of the second type of superhydrophobic coating on aluminum included degreasing the aluminum plates, pulsed nanosecond IR laser treatment (ytterbium fiber laser with a wavelength of 1.064 μm, pulse duration of 50 ns, and repetition rate of 20 kHz), washing with distilled water, drying in an oven at 130 °C, and finally chemical adsorption of the same hydrophobic agent, methoxy-{3[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]propyl}-silane, from solution in dehydrated decane. Multimodal texture having regular surface ripples with a period of a few tens of micrometers (Figure 1d) decorated by nearly spherical aggregates (Figure 1e) composed of nanoparticles (Figure 1f) was characteristic of the latter type of coating. Heat treatment of both types of coatings in the oven at 140 °C in the presence of water vapor after solvent evaporation leads to crosslinking between molecules of the hydrophobic agent24 and the formation of a surface monolayer of fluorooxysilane that is chemically stable to long-term contact with the aqueous phase. The morphologies of both types of surfaces are shown in Figure 1. The microstructure of the samples was studied using a Carl Zeiss Supra 40 VP scanning electron microscope. Micrographs were taken at 5 and 10 kV acceleration voltages using secondary (InLens, SE2) electron detectors. Characterization of the wettability of the coatings was based on contact and rolling angle measurements. We used digital video image processing of sessile droplets to analyze the droplet shape parameters. The homemade experimental setup for recording optical images of sessile droplets and the software for the subsequent determination of droplet parameters using the Laplace curve-fitting routine were described earlier.25 In order to characterize the wetting of different coatings, initial contact angles for 10−15 μL droplets were measured on five different surface locations for each sample. For the measurement of the rolling angle, 10 μL droplets were deposited on the surface. After the initial droplet shape was
Table 1. Contact (CA) and Rolling (RA) Angles on Different Samples Used for Experiments with Drops Cooling and Freezing sample 1: superhydrophobic coating on anodized aluminum
sample 2: superhydrophobic coating on laser-treated aluminum
CA, deg
RA, deg
CA, deg
RA, deg
167.5 ± 2.6
7.6 ± 3.3
170.5 ± 1.6
1.3 ± 0.5
The experiment for the analysis of the temperature behavior of a water droplet contact angle and contact diameter was organized as follows. The droplet was deposited on the substrate inside a doublewalled cell. The water vapor saturation inside the cell at temperatures above 0 °C was achieved by pouring a 2 mm water layer on the bottom of the cell, which had a large water evaporation area. After the cell was sealed and the vapor phase was equilibrated in the cell for 3 h, the temperature in the environmental chamber was lowered at a rate of 1°/h.
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RESULTS AND DISCUSSION The two types of superhydrophobic coatings discussed above were used to study the evolution of droplet shape parameters during droplet cooling and freezing. The temperature behavior of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface fabricated by laser treatment (sample 2) is shown in Figure 2a. As follows from the data in Figure 2a, weak deterioration of the contact angle is observed during droplet cooling. The decrease in contact angle in the temperature interval from 20 to −5 °C is accompanied by an increase in contact diameter, indicating that in this temperature interval we are dealing with an advancing contact angle with weak descent defined by the temperature dependence of surface forces.26 The diminution of droplet volume in the first 28 h of the experiment did not exceed 1%. However, at lower temperatures, the decrease in relative vapor pressure above the ice layer on the bottom of the cell with respect to the supercooled water droplet (Figure 2b) resulted in an intensification of droplet evaporation, causing a decrease in the droplet contact diameter. A more rapid decrease in contact angle in the interval from −5 to −12 °C indicates that the transition from advancing to receding contact angle takes place because of droplet evaporation. Further droplet cooling leads to weak deterioration of the receding contact angle until the droplet freezes. The total diminution of droplet volume for the whole cooling time from room temperature until droplet freezing did not exceed 5%. Because nucleation in supercooled water droplets is a stochastic process, the freezing temperature for individual droplets varies over a wide range. The behavior shown in Figure 2a is typical of this type of superhydrophobic coating, and the difference from sample to sample was associated with the final freezing temperature. The freezing event is easily detected by the disappearance of the bright spot of transmitted light on the droplet image upon freezing, followed by the formation of a frozen cap in the apex of the droplet (Figure 3a− e). In our experiments for freezing temperatures in the range of −15 to −21 °C, the receding water contact angle varied from C
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vapor pressure in the vicinity of the droplet, as determined by the actual aqueous phase temperature.31 The local excess in the vapor pressure compared to the saturated values characteristic of the substrate temperature results in the desublimation of supersaturated vapors inside the gap between the droplet and the substrate and also frost deposition on the substrate under the drop. It is interesting that in the case of a superhydrophobic substrate and nearly saturated ambient atmosphere, no water microdroplet condensation was observed, in contrast to the findings for hydrophilic substrates.22 The appearance of an iced meniscus and frost around the droplet similar to that shown in Figure 3g,h was detected for the process of droplet freezing on top of sample 2. Figure 4a shows the details of meniscus formation versus time for the digitized profile of a sessile droplet, and it may be concluded that the desublimation stage is very fast and is typically completed within a time interval on the order of 1 min (most of the meniscus is formed within 20−30 s). Very rapid spatial relaxation of supersaturated vapor pressure to the values determined by the temperature of the frozen water layer on the bottom of the cell is proven by the fact that the size of the frost halo is very limited and is substantially less than the droplet diameter. It is worth noting that in the first stage of freezing (during 1 s), the contact angle discontinuously decreased slightly while the droplet edge moved moderately upward (dotted magenta lines) and remained essentially unchanged over the next several hours of exposure of the frozen droplet at constant negative temperature and saturated vapors. This indicates that, on the one hand, during freezing, the capillary condensation initiated by short-term droplet heating causes an increase in droplet contact diameter associated with the growth of the meniscus. On the other hand, vapor desublimation between the asperities of the superhydrophobic coating leads to the transition from the heterogeneous to the mixed heterogeneous/homogeneous regime, which is illustrated by an abrupt but not very essential shift in the iced droplet edge. The increase in the ice/substrate contact area associated with the formation of an iced meniscus and frost halo under the droplet can be considered to be the main physical reasons for the enhancement of the actual adhesion strength of the frozen droplet to the superhydrophobic coating. At the same time, it should be stressed that neither the meniscus nor the frost halo corresponds to the stable state of the system represented by the iced droplet on top of the superhydrophobic coating. The spontaneous evolution of the frozen meniscus and the frost halo at the droplet edge with time of exposure to nearly saturated vapors, as can be seen in Figure 4b, indicates the sublimation of water from both the iced meniscus and the frost halo and the tending of the droplet contact diameter to the value characteristic of the droplet just before freezing. This is a long-range process that proceeds over many hours. However, it will favor a decrease in the actual adhesion strength of the iced droplet and will facilitate the detachment of the droplet under an external load.
Figure 2. (a) Variation of the contact diameter (blue circles) and the contact angle (red triangles) of a water droplet deposited on sample 2 with a superhydrophobic surface fabricated by laser treatment as a function of temperature during droplet cooling from 20 °C until droplet freezing at −17.3 °C. (b) At positive temperatures, the water layer on the bottom of the experimental cell supports the 100% relative humidity in the cell; however, at negative temperatures, after the freezing of this layer, the vapors in the cell are undersaturated with respect to supercooled liquid water in the droplet. The graph shows the temperature dependence of relative undersaturation, that is, the ratio of equilibrium vapor pressure above ice, Pice, to that above supercooled water, Pscw. The green line was calculated from empirical equations for vapor pressures,27 and points were calculated from experimental data.28
163 to 161°, whereas the contact diameter was nearly the same as for the freshly deposited droplet at 20 °C. The behavior of the contact angle and contact diameter for the superhydrophobic coating obtained by anodic oxidation23 is similar to those described above for the laser-treated aluminum sample, but with a receding angle slightly greater than 168° just before droplet crystallization. We did not obtain contact angle jumps at negative temperatures in a nearly saturated vapor environment on either of the superhydrophobic substrates. (Such jumps, if observed, could be considered to be an indication of capillary condensation between the asperities and a transition from the heterogeneous to homogeneous or mixed heterogeneous/homogeneous wetting regime.) However, the situation is very different during droplet crystallization. Figure 3a−e shows the optical images of a freezing water droplet on top of sample 1 at different times during the crystallization process. The evolution of the peculiarities of droplet shape can be analyzed from the digitized droplet images shown in Figure 3f−h. It follows that droplet crystallization is accompanied by a variation in droplet shape as a whole, the simultaneous growth of an iced meniscus between the droplet and substrate, and frost halo formation in the vicinity of the three-phase contact zone. The appearance of this meniscus and halo is easily understood if one takes into account that the first stage of droplet freezing can be considered to be quasiadiabatic. That is why the release of the heat of freezing results in some increase in the droplet temperature compared to the temperature of the ambient air and substrate. This phenomenon was detected experimentally22,29,30 and leads to a short-term local increase in
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CONCLUSIONS Two types of superhydrophobic coatings with very different textures were used in this study to analyze the evolution of shape parameters of a sessile water droplet during droplet cooling and freezing. The behavior of the contact angle and contact diameter of the water droplet is similar for both coatings in a saturated water vapor atmosphere. Weak descent D
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Figure 3. Images of a droplet on the surface of sample 1 (superhydrophobic coating on anodized aluminum): (a) liquid droplet just before freezing and (b) 5 s, (c) 15 s, (d) 20 s, and (e) 1 min after the start of freezing. Digitized profiles of droplets at different times (f) and details of menisci at the left (g) and right (h) droplet edges. It is easily seen that the maximum extension of the meniscus is achieved at about 30 s, and after that the meniscus recedes because of sublimation. The orange dashed line in (f−h) indicates the theoretical Laplacian fit to the liquid droplet profile and the position of the substrate.
Figure 4. Evolution of the droplet edge profile and of the iced meniscus and frost halo at the droplet edge upon freezing (a) and with time of exposure to nearly saturated vapors (b). Black solid lines indicate the digitized profile of a liquid droplet just before freezing. Upon freezing, the contact angle discontinuously decreased slightly whereas the droplet edge moved moderately upward (dotted magenta lines) and remained essentially unchanged over the next several hours of exposure at constant negative temperature and nearly saturated vapors. The iced meniscus formed intensively within first minute after the start of droplet freezing, as can be seen in panel (a). Further exposure of the frozen droplet in nearly saturated vapor pressure relative to the iced droplet led to the sublimation of ice from the meniscus and the frost halo from the substrate, as shown on panel (b).
of the contact angle at positive and negative temperatures is a result of the temperature dependence of surface forces. Cooling the cell to below zero leads to the crystallization of the water layer on the bottom of the cell, which was used as a source of saturated vapor inside the cell. The decrease in the relative
vapor humidity above the ice with respect to the supercooled water droplet initiated by this crystallization intensifies droplet evaporation, which is followed by the transition from advancing to receding contact angle. The typical very high values of receding angles detected on both substrates allowed us to E
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(11) Sarshar, M. A.; Swarctz, C.; Hunter, S.; Simpson, J.; Choi, C. H. Effects of Contact Angle Hysteresis on Ice Adhesion and Growth on Superhydrophobic Surfaces under Dynamic Flow Conditions. Colloid Polym. Sci. 2013, 291, 427−435. (12) Arianpour, F.; Farzaneh, M.; Kulinich, S. A. Hydrophobic and Ice-Retarding Properties of Doped Silicone Rubber Coatings. Appl. Surf. Sci. 2013, 265, 546−552. (13) Boinovich, L. B.; Zhevnenko, S. N.; Emelyanenko, A. M.; Goldstein, R. V.; Epifanov, V. P. Adhesive Strength of the Contact of Ice with a Superhydrophobic Coating. Dokl. Chem. 2013, 448, 71−75. (14) Nosonovsky, M.; Hejazi, V. Why Superhydrophobic Surfaces Are Not Always Icephobic. ACS Nano 2012, 6, 8488−8491. (15) Dodiuk, H.; Kenig, S.; Dotan, A. Do Self-Cleaning Surfaces Repel Ice? J. Adhes. Sci. Technol. 2012, 26, 701−714. (16) Meuler, A. J.; Smith, J. D.; Varanasi, K. K.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Relationships between Water Wettability and Ice Adhesion. ACS Appl. Mater. Interfaces 2010, 2, 3100−3110. (17) Kulinich, S. A.; Farzaneh, M. How Wetting Hysteresis Influences Ice Adhesion Strength on Superhydrophobic Surfaces. Langmuir 2009, 25, 8854−8856. (18) Yang, S. Q.; Xia, Q. A.; Zhu, L.; Xue, J. A.; Wang, Q. J.; Chen, Q. M. Research on the Icephobic Properties of Fluoropolymer-Based Materials. Appl. Surf. Sci. 2011, 257, 4956−4962. (19) Zou, M.; Beckford, S.; Wei, R.; Ellis, C.; Hatton, G.; Miller, M. A. Effects of Surface Roughness and Energy on Ice Adhesion Strength. Appl. Surf. Sci. 2011, 257, 3786−3792. (20) Karmouch, R.; Ross, G. G. Experimental Study on the Evolution of Contact Angles with Temperature near the Freezing Point. J. Phys. Chem. C 2010, 114, 4063−4066. (21) He, M.; Li, H.; Wang, J.; Song, Y. Superhydrophobic Surface at Low Surface Temperature. Appl. Phys. Lett. 2011, 98, 093118. (22) Jung, S.; Tiwari, M. K.; Poulikakos, D. Frost Halos from Supercooled Water Droplets. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 16073−16078. (23) Boinovich, L. B.; Emelyanenko, A. M.; Korolev, V. V.; Pashinin, A. S. Effect of Wettability on Sessile Drop Freezing. When Superhydrophobicity Stimulates Extreme Freezing Delay. Langmuir 2014, 30, 1659−1668. (24) Boinovich, L.; Emelyanenko, A. A Wetting Experiment as a Tool to Study the Physicochemical Processes Accompanying the Contact of Hydrophobic and Superhydrophobic Materials with Aqueous Media. Adv. Colloid Interface Sci. 2012, 179, 133−141. (25) Emelyanenko, A. M.; Boinovich, L. B. Application of Dynamic Thresholding of Video Images for Measuring the Interfacial Tension of Liquids and Contact Angles. Instrum. Exp. Tech. 2002, 45, 44−49. (26) Boinovich, L.; Emelyanenko, A. Wetting and Surface Forces. Adv. Colloid Interface Sci. 2011, 165, 60−69. (27) Murphy, D. M.; Koop, T. Review of the Vapour Pressures of Ice and Supercooled Water for Atmospheric Applications. Q. J. R. Meteorol. Soc. 2005, 131, 1539−1565. (28) Spravochnik Khimika (Chemist’s Handbook); Nikolsky, B. P., Ed.; Goskhimizdat: Leningrad, 1962; Vol. 1. (29) Alizadeh, A.; Yamada, M.; Li, R.; Shang, W.; Otta, S.; Zhong, S.; Ge, L.; Dhinojwala, A.; Conway, K. R.; Bahadur, V.; Vinciquerra, A. J.; Stephens, B.; Blohm, M. L. Dynamics of Ice Nucleation on Water Repellent Surfaces. Langmuir 2012, 28, 3180−3186. (30) Yin, L.; Xia, Q.; Xue, J.; Yang, S.; Wang, Q.; Chen, Q. In Situ Investigation of Ice Formation on Surfaces with Representative Wettability. Appl. Surf. Sci. 2010, 256, 6764−6769. (31) Bakanov, S. P. The Effect of Phase and other Transformations at the Surface of Particles on the Thermophoresis of Aerosols. Colloid J. 1995, 57, 731−735.
conclude that at negative temperatures the wetting regime of superhydrophobic substrates remains heterogeneous. The heterogeneity is responsible for the weak adhesion of droplets to a superhydrophobic coating, which was confirmed by the experimental observation of the spontaneous removal of droplets from a horizontal superhydrophobic surface during shaking at negative temperatures. However, upon droplet crystallization, the release of heat of freezing leads to some increase in the actual droplet temperature and a local oversaturation of vapors in the vicinity of the droplet. As a result, the simultaneous growth of an iced meniscus inside the gap between the droplet and substrate and frost deposition in the vicinity of the three-phase contact zone occur within 1 min. These phenomena will be characteristic of superhydrophobic coatings of any nature and follow from the universal phenomenon of the release of the latent heat of phase transition in the crystallizing droplet. However, it may be essentially suppressed if the substrate temperature is kept higher than the droplet and the ambient atmosphere temperature. Another important observation of this study is related to the establishment of a metastable state of the desublimated meniscus and the frost halo around the droplet base. This causes the spontaneous sublimation of water from both the frozen meniscus and the frost halo with time of exposure to saturated vapors. Although this process takes many hours, it will favor a decrease in the actual adhesion strength of the iced droplet and can be successfully used in practice to facilitate the detachment of ice from superhydrophobic surfaces.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was performed with the financial support of the Russian Scientific Foundation (project no. 14-13-01076). REFERENCES
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