Article pubs.acs.org/Langmuir
Cold-Induced Spreading of Water Drops on Hydrophobic Surfaces Faryar Tavakoli* and H. Pirouz Kavehpour Department of Mechanical and Aerospace Engineering, University of California, Los Angeles (UCLA), Los Angeles, California 90024, United States S Supporting Information *
ABSTRACT: Superhydrophobic surfaces are characterized by their peculiarities, such as water-repellent, anti-icing, and freezing-delay properties. Wetting dynamics of deposited water drops on cooling hydrophobic surfaces, which directly affects the aforementioned properties, has not been studied thoroughly. Here, water drops are cooled on different hydrophobic surfaces in a controlled environment. During the cooling process, a significant increase in the drop footprint and decrease in the apparent contact angle are observed because of premature and capillary condensation, followed by thin water film formation adjacent to the solid−liquid−gas line. The water thin film propagates on the hydrophobic substrates radially away from the trijunction, followed by spreading of the drop on the film, which was experimentally validated through high-speed visualization. In addition, the roles of physical variables, such as the substrate temperature, humidity of surrounding air, types of hydrophobic surfaces, surface roughness, and drop volume, on post-spreading shape are investigated experimentally.
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INTRODUCTION Superhydrophobic surfaces are universally used for their unique self-cleaning properties, as water droplets roll along these surfaces rather than spread. In addition, there have been many studies in the past decade on the capability of hydrophobic surfaces to induce a significant delay on freezing initiation or reduce the ice adhesion at extremely cold conditions.1−10 Many factors, such as insulating properties of these surfaces,5 less probability of heterogeneous nucleation, and reduction of the water−solid interfacial11−13 area, have been attributed to these unique features. In practice, the efficiency of the hydrophobic surfaces under conditions of extreme humidity, airflow,8 particle diameter of the hydrophobic surface,14 icing/deicing cycles,15 roughness level,16 and frost formation9 is critically questioned. More specifically, geometrical characteristics of the water drop on the hydrophobic surfaces, i.e., contact angle and base diameter, which determine icephobicity and freezing delay qualities, have been explored. Huang et al. unveiled the strong relationship between the contact angle and base diameter of the water drops with the droplet freezing time.17 According to their study, the larger the contact angle, the longer the freezing time. Despite the obvious influence of the contact angle and base diameter of the drop on the ice adhesion18−21 and freezing time17,22,23 of the supercooled drops, a systematic investigation on wetting dynamics of water drops on hydrophobic surfaces during the cooling stage, prior to freezing initiation, has not been conducted. Few studies about the thermal stability of hydrophbic surfaces have been published. Some claimed24,25 that decreasing the temperature gradually alters the wettability of hydrophobic surfaces, whereas, on the contrary, others26,27 © XXXX American Chemical Society
reported that the hydrophobicity remained relatively stable at temperatures as low as −5 °C. When liquid drops are deposited on hydrophobic surfaces and then cooled to the temperatures below room temperature (RT), net force balance at the trijunction predicts, in an ideal scenario, that introducing coldness to the solid substrate will increase the surface tension of the liquid/gas interface and cause the drop to recede, leading to a smaller base diameter and higher contact angle (Figure 1). The present study aims to investigate the drop dynamics caused by cooling hydrophobic substrates and unveil the underlying mechanism. The coldinduced dynamics of water drops on hydrophobic surfaces is investigated, and the effects of the substrate temperature,
Figure 1. Predicted dynamics of a water drop upon cooling hydrophobic substrates. Theoretically, cooling of a water drop from the substrate should augment the surface tension of the liquid drop, leading to recession of the water drop. Received: September 10, 2014 Revised: January 5, 2015
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from 24 to −10 °C yields roughly 0.19 °C/s; however, while reaching lower temperatures of −20 °C, cooling rate deviates from linearity and decreases in time. No tangible fluctuations of the temperature are observed after reaching the assigned temperatures. Temperature measurements of the Peltier surface using a k-type thermocouple and Forward Looking Infrared (FLIR A655SC) showed 0.2 °C higher values than the readings of the Peltier temperature controller. A commercially available instrument, DSA100 by Krüss GmbH, is used for recording the droplet spreading dynamics and subsequent evaluation. The basis for the determination of the contact angle and base diameter is the image of the drop on the solid surface. In the DSA100 program, the actual drop shape and the contact line with solid are first determined through a greyscale edge-detection technique; i.e., the software calculates the root of the secondary derivative of the brightness levels to acquire the point of greatest change of brightness. Then, the profile of a sessile drop in the region of the baseline is fitted to a polynomial function. Afterward, the slope of the three-phase contact point at the baseline is first determined and used to measure the contact angles. Consequently, the base diameter is defined as a distance between contact points.
humidity, types of hydrophobic surfaces, drop volume, and surface roughness on the spreading are explored. The solid− liquid−gas line (trijunction) during the spreading is monitored closely using a high-speed camera to track the movement closely. To the best of our knowledge, no previous report has been published that inquires the temporal evolution of the sessile water drops upon cooling hydrophobic surfaces from RT to subzero temperatures, with the addition of some of the aforementiond parameters, such as relative humidity (RH) and drop volume.
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EXPERIMENTAL PROCEDURES AND MATERIALS
Four different kinds of hydrophobic surfaces of WX2100, Fluorothane, silicon pillars, and Teflon, with equilibrium angles of 144° ± 1.8°, 142° ± 1.5°, 149° ± 1.3°, and 108° ± 2.1°, respectively, are used. The silicon pillars are rectangular, and the area fraction of the solid surface in contact with water (ϕs) and roughness factor (r) are 0.25 and 1.15, respectively. The roughness factor value is calculated from
r=
(a + b)2 + 4ac (a + b)2
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RESULTS AND DISCUSSION Water drops of 9 μL with a temperature of 25 °C were deposited on four different hydrophobic surfaces (Teflon, WX2100, Fluorothane, and silicon pillars) to evaluate the effects of surface morphology and static contact angle. Afterward, water drops are cooled to −10 °C with the Peltier element placed under the assigned hydrophobic surfaces. During the cooling stage, the water drops depin from the substrate and started spreading in stick−slip mode, contrary to the ideal scenario depicted in Figure 1. Evolution of the drops was recorded by a camera, with recording speed of 100 fps, in the DSA100 machine. Figure 3 shows the side view of a water
(1)
where a is the pillar width, b is the pillar pitch, and, c is the pillar height. The values of a, b, and c for the silicon pillars are 75, 75, and 10.5 μm, respectively. Both WX2100 (aerosol coating) and Fluorothane (solution coating) are applied to the surface of the VWR microcover glasses. It is worth mentioning that the thickness of applied WX2100 spray and applied Fluorothane solution on the cover glasses do not change the wetting characteristics. The hydrophobic surfaces, on which water drops are deposited, are placed into the DSA chamber in which both temperature, using the Peltier element, and humidity, using nitrogen gas, are accurately controlled. The Peltier element is situated in the DSA100 to accurately observe the dynamics. A Peltier element is a thermoelectric device that transfers heat from one of its sides to the other when electricity is applied. Amplifying the voltage across the thermoelectric heaters increases the temperature of the Peltier element. The cold side is then brought in contact with the substrate and cools it to a chosen temperature after the water droplet is placed on it. Cooling rates are kept constant by the circulation of coolant water with a predetermined temperature around the hot side of the Peltier element. Because we are dealing with temporal evolution of water drops on a cooling Peltier target, the temperature readings of the Peltier element during the cooling stage is crucial. Figure 2 shows the temperature of the Peltier element versus time for different cooling ranges from 25 to 10, −10, and −20 °C. For temperatures down to −10 °C, the cooling rate can be considered to be constant. The cooling rate of the Peltier element
Figure 3. Side view of cold-induced spreading of a water drop with a volume of 9 μL on a WX2100-coated glass. The substrate is cooled from 25 to −10 °C. The initial drop diameter (frame a) and postspreading diameter (frame b) are 1.78 and 2.48 mm, respectively. The vertical black line is a hypodermic needle used for depositing the drops.
drop with volume of 9 μL cooled to −10 °C on WX2100coated cover glass prior to spreading (Figure 3a) and after spreading (Figure 3b). The black vertical line in Figure 3a is the injection needle used to deposit the water drops. Figure 4 shows the apparent base diameter of the water drops versus time for different hydrophobic surfaces cooled to −10 °C. At t = 0, substrate cooling from RT to the assigned temperatures begins. The only exception is for a water drop on Teflon that rests at a 108° equilibrium contact angle, in which the drop remains immobile. Post-spreading diameters on WX2100- and Fluorothane-coated surfaces are roughly 33% larger than the initial diameter.
Figure 2. Temperature of the Peltier element situated in the DSA100 machine versus time during cooling cycles. B
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irreversible; i.e., the water drops do not recede when the substrate temperature is raised to RT from lower temperatures. The trijunction region of the spreading drops upon cooling hydrophobic targets is monitored using a high-speed camera. Figure 6 shows the initial and post-spreading of the water drop
Figure 4. Apparent base diameter of water drops versus time on different hydrophobic surfaces cooled to −10 °C at 18% RH. Initial/ static contact angles of the water drops for different surfaces are depicted in the legend.
Figure 6. Frames from the cold-induced spreading of a water drop on cold solid substrates. Panel a shows the trijunction region prior to spreading and cooling initiation, whereas panel b shows the drop after the spreading. Premature condensation followed by thin-film formation and propagation at the trijunction initiates the drop spreading.
To evaluate the role of the substrate temperature, water drops with a constant volume of 9 μL are cooled from 25 °C to assigned temperatures of 15, 10, 0, −10, and −20 °C on a WX2100-coated substrate at 18% RH. During the cooling process, the drops start spreading gradually over the solid substrate. No variation of the contact angle and base diameter is observed for temperatures ranging from RT to 15 °C; however, while cooling below 15 °C, the final diameter and apparent contact angle of the drop are dependent upon the substrate temperature (Figure 5). The water drops cooled to −20 °C
after being cooled to −10 °C on a WX2100-coated surface. The camera has been adjusted to +4° above the horizon to clearly view the trijunction region. The premature condensation followed by thin-film formation and propagation at the trijunction is observed in Figure 6. During the thin-film propagation, the water drops start coalescing with the thin film and the distinct microcondensed drops, resulting in spreading initiation (refer to the Supporting Information). After the final growth of the thin film and microcondensed drop enlargement around the trijunction, the water drop stops spreading. The main mechanism for this cold-induced enhanced spreading is due to premature and capillary condensation, followed by thin-film formation and propagation around the trijunction. Capillary condensation is a ubiquitous phenomenon in nano- and even microscales28 involving confined regions. In capillary condensation, vapor condensation occurs below the equilibrium vapor pressure because of higher surface interactions, such as capillary and van der Waals (vdW) forces, at confined areas. The strong influence of the RH and temperature on capillary condensation in contained areas is explained by the Kelvin equation.29 In these constricted regions, condensation transpires at higher temperatures and lower humidities than equilibrium values. Confined regions in the context of capillary condensation can refer, however not limited, to the sharp cracks,30 mesoporous media,31,32 and contacts between planar and spherical surfaces.33,34 In the case of water drops on cold hydrophobic surfaces, trijunction adjacent between a water drop and the hydrophobic solid substrate is considered to be convexly confined, except for Teflon with a contact angle of 108°, on which spreading does not occur (Figure 4). At this region, condensation transpires at higher temperatures and lower RH values compared to equilibrium. Vapor molecules accumulate at the trijunction, and because of increased vdW interactions at this location, premature condensation occurs. The condensation develops in the form of thin-film propagation away from the drop with distribution of satellite droplets (refer to the Supporting Information). Moreover, some simulations35,36 reported that confined fluid between a solid sphere and planar solid is stable only in the shape of a concave meniscus. The concavity of the
Figure 5. Apparent contact angle of 9 μL water drops versus time on a WX2100-coated substrate cooled by the Peltier element to temperatures of 15, 10, 0, −10, and −20 °C at 18% RH. The inset shows the drop diameter versus time for the associated experiments.
finally froze after the post-spreading geometry from cooling is reached. During and after solidification, no change in the drop base diameter and contact angle is observed. Further cooling beyond −20 °C does not lead to a larger post-spreading base diameter. The elapsed time to spreading initiation is fairly independent from the assigned temperature; however, colder substrates amplified spreading rates with a larger post-spreading footprint (Figure 5). The depinning followed by spreading occurred axisymmetrically rather than spreading in one direction. The cold-enhanced spreading of water drops is C
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and evaporation from the water drop both contribute to the dynamics. To examine the effect of roughness-induced hydrophobicity on cold-induced spreading, a simple and inexpensive procedure to fabricate hydrophobic surfaces from Teflon, introduced by Nilsson et al.38, is followed. In this method, Teflon surfaces are sanded using different grit designations and subsequent cleaning of solid Teflon. A range of commercial sandpapers with ISO/FEPA designations between 120 and 2000 is used to introduce roughness to smooth Teflon surfaces. Grit sizes correspond to the size of the particles and distance of the abrading particles embedded on the sandpaper. The higher the grit number, the smoother the sandpaper. Each Teflon surface is held stationary and sanded by hand for 25 s. After each sanding, samples were cleaned thoroughly in acetone, methanol, and deionized water. After drying under the air gun, the samples were heated to remove the remaining moist from the roughened Teflon surfaces. The water drops with a volume of 9 μL are deposited on the Peltier element situated in the DSA machine. A higher static contact angle for rougher surfaces is caused by a decrease in contact angle hysteresis. It is worth mentioning that the static contact angle of the drops on roughened Teflons, prior to cooling, is not dependent upon additional sanding, in terms of both force and direction of the sanding. The water drops are then cooled to −10 °C by the Peltier element. Figure 8 shows the spread factor, ratio of the post-spreading drop diameter (Df) to the initial diameter (D0), versus the grit
microscale meniscus at trijunction renders the spreading water drops on hydrophobic surfaces even more favorable. Furthermore, a recent study37 on the evaporation of water drops showed that vapor concentration around the trijunction of a water drop on hydrophobic surfaces is maximum. This high concentration of vapor around the trijunction further confirms the premature condensation on the substrate, which leads to spreading inception. In addition to water, ethylene glycol drops are deposited on WX2100-coated surfaces with an initial contact angle of 135° and cooled to subzero temperatures to observe any change in geometry. The liquid film formation and subsequent spreading of the drops have not transpired, which leads to the fact that the condensation and drop dynamics thereafter at the trijunction are mostly dominated by the water liquid itself rather than directly from the vapor-phase condensation surrounding the water drop. Because the cold-induced dynamics is clearly driven by the premature condensation at the trijunction, the effect of RH needs to be studied. RH is defined as the ratio of the partial pressure of water vapor to the saturated vapor pressure of water at any given temperature. Humidity, in general, may have two distinct effects in spreading of the water drops on solid targets: (i) condensation on the solid surface, which may render the solid substrate prewetted, facilitating further spreading, and (ii) evaporation rate of the drop, especially at the contact line region. To elucidate the effect of humidity on cold-induced spreading, for a constant change in the substrate temperature (from 25 to −10 °C), humidity was adjusted using a nitrogen flux into the closed chamber of the DSA100 apparatus. First, RH is stabilized in the chamber, and then cooling of the Peltier element is initiated. The spreading of the water drops showed a significant dependency upon humidity variations. Figure 7
Figure 8. Dimensionless diameter, post-spreading diameter to initial diameter, of the water drops cooled to −10 °C versus the grit size used for sanding the Teflon surface. The lower grit size of sandpaper corresponds to coarser roughness. Figure 7. Apparent base diameter of 9 μL water drops versus time for RHs of 6.5, 18, and 48% on a WX2100-coated cover glass cooled to −10 °C.
size. Six data points are collected for each designated sanded Teflon. A larger standard deviation of static contact angles is observed for rougher surfaces. The static contact angle of the water drops on 2000 grit sanded Teflon is the same as the Teflon itself, meaning that extremely fine roughness does not impart extra hydrophobicity to the Teflon surface. No substantial change in the contact angle or base diameter is observed for water drops deposited on 1000 and 2000 grit sanded Teflon. The ratio of the post-spreading diameter versus the initial diameter increases with the surface roughness, except for 120 grit size. The change of behavior from 220 to 120 grit size is attributed to extra coarse structure that acts as a resisting force to inception and continuation of the spreading. Figure 9
shows the diameter of the water drop on hydrophobic surfaces for different RH levels of 6.5, 18, and 47%. Higher humidity facilitates the spreading onset and increases the final diameter of the water drop. At RHs of 6.5, 18, and 47%, the drop footprint becomes 16, 31, and 40% larger after the cold-induced spreading. Higher humidity results in a larger driving force for capillary condensation at the trijunction and facilitates condensation growth. In the growth phase of the condensates around the trijunction, condensation from the surrounding air D
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Table 1. Elapsed Time to Depinning of Water Drops on a WX2100-Coated Surface for Different Drop Volumes drop volume (μL)
drop diameter (mm)
0.4 4.1 39 66.9 180.8 556
0.47 1.12 3.13 3.90 6.556 11.22
elapsed time to depinning (s) 126.7 67.6 37.5 32.05 42.57 36.3
± ± ± ± ± ±
4.5 4.2 3.3 3.4 2.1 2.8
fairly the same. For the water drops smaller than the capillary length, the elapsed time to drop depinning increases, because surface forces become more dominant in this length scale. The capillary length is defined as Lcap = (σ/ρg)1/2, where σ is the surface tension, ρ is the liquid density, and g is the gravitational acceleration. For deionized water, the capillary length is 2.7 mm. For smaller drops in the micrometer range, the vdW forces become controlling. Both water drops with base diameters of 0.48 and 1.14 mm depinned at longer time scales with respect to the drops larger than the capillary length. The importance of the pinning effect for isothermal spreading of small drops is experimentally and numerically verified by Perez et al.39 Independency of elapsed time to depinnig of the water drops to various volume sizes with the same contact angle, except for the water drops smaller than the capillary length at which vdW forces are important, signifies the effect of surface forces, such as capillary condensation, around the confined region rather than bulk forces depending upon the drop volume. It is already shown that the initial contact angle of the drop partially dictates the cold-induced dynamics of the water drops. However, adjustment of the initial contact angle is provided by introducing roughness to the surface. To disentangle the influence of roughness and solely evaluate the effect of the initial contact angle, we deposited single water drops on a WX2100-coated surface and decided to impart an uneven distortion to the drops using the 0.5 mm injection needle. This distortion causes the drop to attain non-equal right and left contact angles (Figure 11a). Then, cooling of the substrate using the Peltier is carried out. During cooling of the substrate from 25 to −10 °C, the water drop depins from the larger contact angle, in this case left contact point, and starts
Figure 9. Spread factor of the water drops cooled to −10 °C on sanded Teflon surfaces versus initial contact angles (radians). Higher contact angles correspond to rougher surfaces introduced by sanding.
shows the relationship between the spread factor and initial contact angles of the sanded Teflons. The correlation shows that spread factor soars rapidly (with a power value of 11) with a minute increase in roughness-induced hydrophobicity. To evaluate the impact of body forces on the cold-induced spreading, different volumes of the water drops are deposited on the WX2100-coated surface and cooled to −10 °C using the Peltier element. The diameter of the water drops ranges from 0.47 to 11.22 mm with the same initial contact angle of 144° ± 1.8°. Figure 10 depicts the water drop diameter upon cooling
Figure 10. Apparent diameter of the water drops upon cooling WX2100-coated surfaces versus time for different drop volumes.
the WX2100-coated surface versus time for multiple drop volumes. Each curve represents an average of six experiments for the specific drop volumes shown. The rate of data acquisition is 2 Hz; however, for better distinction between data sets, intermittent data points are selectively removed. Table 1 lists the volume and base diameter of the drops with their associated elapsed time to depinning of the drops during the cooling cycle. The elapsed time to depinning can be evaluated visually from the Supporting Information and also the drop diameter versus time plot (Figure 10). At a constant humidity and cooling cycle, the spreading kinetics and initiation at a relatively large length scale is only a function of the contact angle (Table 1 and Figure 10). The elapsed time to spreading initiation (depinning) for relatively medium or big size drops is
Figure 11. Cold-induced spreading of a non-axisymmetric water drop contorted by the injection needle on a WX2100-coated cover glass cooled from RT to −10 °C. The depinning of the drop initiates from the larger, in this case left, contact angle with a minute movement at the right contact angle. Panel a shows the drop prior to cooling initiation, whereas panel b depicts the drop after spreading completion. E
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hydrophobic surfaces depin and start to spread (up to 33% of the initial diameter) on cooling hydrophobic surfaces. This cold-induced spreading is found to be strongly dependent upon the substrate temperature, RH of surrounding air, and initial contact angle of the drop. From the physical parameters studied and visually monitoring the trijunction region during spreading, the spreading is due to premature condensation and formation of a liquid rim at the trijunction, followed by the film propagation radially away from the drop. During the growth phase of the thin film, the drop depins and starts spreading gradually on the film. Lower temperatures of the hydrophobic substrate resulted in more spreading of the drops; however, different designated temperatures have not affected the time to spreading initiation substantially. A higher humidity of surrounding air facilitated the onset of spreading with a larger final footprint of the drops. Introducing roughness to the hydrophobic surfaces lead to a higher post-spreading diameter; however, rougher surfaces impede cold-induced spreading. The water drops with a smaller base diameter than the capillary length spread less because of the pinning effect that stemmed from strong capillary and even vdW forces at the microscale. To evaluate the sole dependence of cold-induced spreading upon initial contact angle, the deposited water drop on a hydrophobic surface is distorted using an injection needle to alter the symmetry of the water drop. This contortion by the needle causes the drop to attain different angles at right and left contact points. Spreading transpires at the contact point, with larger contact angles confirming the paramount effect of the initial contact angle on cold-induced dynamics. The results corroborate the influence of many parameters, such as the substrate temperature, RH, surface roughness, initial contact angle, and drop volume, on the temperature-induced spreading of water drops upon cooling hydrophobic surfaces. This will be of great importance in the design, evaluation, and development of superhydrophobic materials for freezing-delay and anti-icing applications, such as aircrafts, antennas, wind turbines, and power lines. The interplay and competition of both direct condensation from the vapor phase and condensation from the “mother” water drop itself on its dynamics is yet to be quantified. It is hoped that our experimental results and provided explanation presented in this work provide some insight into the cold-induced spreading of water upon cooling hydrophobic surfaces and stimulate more studies on this subject.
spreading (Figure 11b). Figure 12 shows the contact angles of both right and left contact points of the water drop shown in
Figure 12. Contact angle measurements of right and left contact points during cold-induced spreading of the water drop shown in Figure 10.
Figure 10 during the cold-induced spreading. Both contact angles decrease during the cold-induced spreading. The difference between right and left contact angles at the initial condition is 17.9°, whereas preferred spreading diminishes this difference to 7.8° at the final post-spreading diameter. Figure 13
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Figure 13. Relative right and left contact point movements versus time for the experiment in Figure 9. ΔD is the base diameter change with respect to the initial position of the contact points.
ASSOCIATED CONTENT
* Supporting Information S
Video of a water drop coalescing with the thin film and distinct microcondensed drop around the trijunction during cooling stage of the hydrophobic drop resulting in spreading initiation. This material is available free of charge via the Internet at http://pubs.acs.org.
shows the relative right and left contact point movement in time for the experiment shown in Figure 11. The left contact point is displaced 0.8 mm, whereas trivial movement is observed at the right contact point. The preferred motion from the larger contact angle of the drop for the same surface confirms the paramount effect of the initial contact angle on cold-induced dynamics. Moreover, this means that the premature condensation and liquid film formation around the larger contact angle precedes the smaller contact angle and confirms the effect of convexity of the trijunction on the enhanced condensation and subsequent spreading inception.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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CONCLUSION AND OUTLOOK We studied the cold-induced dynamics of sessile water drops on cooling hydrophobic substrates. The water drops on cooling
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DOI: 10.1021/la503620a Langmuir XXXX, XXX, XXX−XXX
