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Meltwater evolution during defrosting on superhydrophobic surfaces Fuqiang Chu, Xiaomin Wu, and Lingli Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16087 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Applied Materials & Interfaces

Meltwater evolution during defrosting on superhydrophobic surfaces

Fuqiang Chu, Xiaomin Wu*, Lingli Wang

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Key Laboratory for CO2 Utilization and Reduction Technology, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China *Corresponding author, Tel: +86-10-62770558, E-mail: [email protected]

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Abstract: Defrosting is essential for removing frost from engineering surfaces, but some fundamental issues are still unclear, especially for defrosting on superhydrophobic surfaces. Here, defrosting experiments on prepared superhydrophobic surfaces were conducted with the meltwater evolution characteristics investigated. According to the experiments, the typical meltwater evolution process on superhydrophobic surfaces can be divided into two stages: dewetting by edge curling and dewetting by shrinkage. The edge curling of a meltwater film is a distinct phenomenon and first reported in this work. Profiting from the ultralow adhesion of the superhydrophobic surface, the edge curling is mainly attributed to two unbalanced forces (one in the interface between ice slurry layer and pure water layer, and another in the triple phase line area) acting on the layered meltwater film. During the multi-meltwater evolution process, the non-breaking of chained droplets on superhydrophobic surfaces is also an interesting phenomenon, which is controlled by the interaction between the surface tension and the retentive force due to the contact angle hysteresis. An approximate criterion was then developed to explain and judge the status of chained droplets, and experimental data from various surfaces have validated the effectiveness of this criterion. This work may deepen the understanding of the defrosting on superhydrophobic surfaces and promote the anti-frosting/icing applications in engineering. Keywords: defrosting; superhydrophobic; meltwater; edge curling; non-breaking; chained droplets;

1. Introduction Due to their special operation conditions, many industrial equipment and components, such as heat exchangers, wind turbines and airfoils, suffer serious frost/ice problems. 1-3 The accumulated frost/ice reduces the efficiency of these equipment and causes safety accidents, resulting in great economic losses and even casualties. 4 People want to solve the frost/ice problems and have made many efforts to restrain the frost/ice growth and accumulation, 5-6 but excessive energy consumption is undesired when solving these problems. In recent years, the use of superhydrophobic surface with great water repellency is indeed a good idea. Due to its large water contact angle and small contact angle hysteresis, the superhydrophobic surface has large nucleation energy barrier for ice nucleation, small ice−surface contact area for heat transfer and weak frost/ice adhesion, which can reduce frost/ice accretion effectively. 7-10 In addition, researchers have reported that, triggered by spontaneous coalescence, the droplets easily start self-propelled droplet movements such as jumping and sweeping on the superhydrophobic surface, 11-15 which were thought to be a new way to restrain frost/ice, as the subcooled droplets may be able to be repeatedly removed from the surface before ice nucleation occurs. 10, 16-19 2 / 14


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But when considering the harsh environmental conditions such as lower temperatures, higher humidity and longer running time, even the superhydrophobic surface will finally be covered by frost. 16, 19, 20-23 At this time, defrosting on the superhydrophobic surface is essential for removing frost and needs to be understood. Recently, Boreyko et al. conducted defrosting experiments on a horizontal superhydrophobic surface and observed that unstable slush film composed of meltwater and frost crystal could dewet into Cassie-state droplet, which was highly mobile and able to roll away at small tilt angle. 24 Chen et al. observed that the frost meltwater exhibited a relative low fracture density and became a large spherical water droplet at the end of defrosting on horizontal nanograssed superhydrophobic surfaces. 25 The self-propelled behavior of melting droplets during defrosting was also reported, 22, 26-27 and many researches have confirmed the excellent meltwater drainage performance on superhydrophobic surfaces. 28-30 However, some fundamental issues are still unclear. For example, how does the meltwater film dewet into a spherical droplet? Why does the meltwater droplet exhibit a low fracture rate? In this work, defrosting experiments on prepared superhydrophobic surfaces were conducted, during which some fundamental but important and interesting phenomena were observed with above issues addressed. 2. Experimental Section Experimental surfaces The experimental surface is an Al-based superhydrophobic surface fabricated by the chemical etching-deposition method reported in our previous work. 31 Figure 1(a) shows the SEM images of the experimental surface that the surface is covered by flower-like hierarchical structures with 5−10 µm microstructures and these microstructures are aggregations of spindly nanoscale particles. The flower-like hierarchical structures on the experimental surface are key points for the surface to maintain excellent superhydrophobicity. 31-32 Figure 1(b) shows the wettability measurement of the experimental surface. The measured apparent contact angle (CA or θ0) of a 2 µL deionized water droplet by a contact angle goniometer is 160.0o ± 0.5o. The advancing contact angle (θa) and receding contact angle (θr) are 162.2o ± 1.0o and 158.3o ± 1.0o, respectively, with the contact angle hysteresis (CAH) being about 4.0o, indicating the ultralow surface adhesion of the superhydrophobic surface.

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Figure 1 (a) SEM images of experimental superhydrophobic surface: the surface is covered by flower-like hierarchical structures with 5−10 µm microstructures and these microstructures are aggregations of nanoscale particles; (b) Contact angle measurements of experimental superhydrophobic surface: the apparent contact angle is measured to be 160.0o and the contact angle hysteresis is about 4.0o. Experimental System and Conditions The experimental system and apparatus used in this research were the same as that in Refs. 22, 31. The experimental surfaces were horizontally placed on the cold side of a thermoelectric cooler with the experiments performed in a closed laboratory. Before defrosting experiments, frosting experiments were first conducted to accumulate moderate frost. During the frosting experiments, the humid air temperature was 18.0 ± 1.0°C with 80 ± 5% humidity, the experimental surface temperature was − 16.0 ± 0.5°C, and the frosting duration time was 30 min (the frost thickness was nearly 1 mm when the frosting experiments finished). After the frosting experiments, the power of the thermoelectric cooler was shut off and the heat was instantly transferred from the hot side of the thermoelectric cooler to the experimental surface and then to the frost. When the frost temperature rose above 0°C and the nucleation energy barrier of liquid phase was overcome, the frost began to melt. The detailed working conditions during the frosting and defrosting experiments with the frost melting time were shown in Figure 2. 100 80 Value

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RH (%) Tair (°C)

40

Tsur (°C)

Frost melting began

20 0 -20

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Figure 2 Detailed working conditions during the frosting and defrosting experiments on the experimental superhydrophobic surfaces. During the frosting experiments, the humid air temperature (Tair) was 18.0 ± 1.0°C with 80 ± 5% humidity (RH), the experimental surface temperature (Tsur) was − 16.0 ± 0.5°C, and the frosting duration time was 30 min. At the end of the frosting experiments, the power of the thermoelectric cooler was shut off and the surface temperature began to rise rapidly. When the surface temperature rose slightly above 0°C, the frost melting began. 3. Results and Discussion Meltwater Evolution and the Edge Curling Figure 3(a) shows the typical meltwater evolution from a meltwater film to a droplet during defrosting on the superhydrophobic surface. An unstable meltwater film first dewets in a way of edge curling (marked by cambered arrows), forming into a slender meltwater column (at 4 s in Fig. 3(a)), which then shrinks and finally becomes a spherical droplet. Figure 3(b) is the schematic diagram of the typical meltwater evolution process, during which two main stages can be divided: dewetting by edge curling and dewetting by shrinkage. Figure 3(c) is the schematic diagram of multi-meltwater evolution process that several connected meltwater films dewet into chained droplets and these chained droplets coalesce into one large droplet instead of breaking up (see Fig. S-1 in the supporting information for experimental images of multi-meltwater evolution).

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Figure 3 Meltwater evolution during defrosting on superhydrophobic surfaces. (a) Experiment images (see video S-1 in the supporting information for video display): meltwater films first dewet into slender meltwater columns in a way of edge curling (showed by cambered arrows), then the meltwater columns dewet into spherical droplets via shrinkage (showed by straight arrows); (b) schematic diagram of typical meltwater evolution process showed in (a); (c) schematic diagram of multi-meltwater evolution process that several connected meltwater films dewet into chained droplets and these chained droplets coalesce into one large droplet instead of breaking up finally (see Fig. S-1 in the supporting information for experimental images) It should be noted that the edge curling is a distinct phenomenon occurred during the meltwater evolution process on superhydrophobic surfaces. In Fig. 4, the mechanism of edge curling is explained. Fig. 4(a) is the original frost layer on superhydrophobic surfaces. Before melting, the temperature of the entire frost layer (including the upper frost surface and the lower frost surface) is below 0oC.33 After the power of the thermoelectric cooler was shut off, the part of frost whose temperature first rises above 0oC will first melt. Because of the much larger thermal conductivity of the metal superhydrophobic surface than that of the air, the thermal resistance between the thermoelectric cooler and the lower frost surface is much less than that between the air and the upper frost surface, so the heat transfer from the thermoelectric cooler to the lower frost surface via the metal surface is much more quickly. As a result, the temperature of the lower frost surface rises above 0oC first, then the heterogeneous nucleation of liquid phase takes place in the lower frost surface, and the frost there melts first. Actually, the freezing droplet on metal surfaces also melts from its base to its tip, which is attributed to the same reason. 34-35 In addition, because the ice density is less than the water density, the ice tends to stay at the upper meltwater surface. Therefore, the meltwater film is layered with ice slurry in the upper layer and pure water in the lower layer, as shown in Fig. 4(b). According to the researches on the surface tension of liquid marbles, 36 repulsive capillary forces between particles decrease the effective surface tension on the marble upper surface compared to that of pure liquid while attractive capillary forces increase it. 37-38 Since the ice particle in the ice slurry is hydrophilic with quite attractive capillary force, the ice slurry may have a larger effective surface tension than pure water, i.e., γlg, ice slurry > γlg, resulting in unbalanced surface force (Fsur) in the interface between ice slurry layer and water layer (shown in Fig. 4(b)). What is more, at the initial stage of melting, the immediate contact angle in the triple phase line area is usually less than the static contact angle, causing non-zero resultant force of the three surface tension (gas-liquid surface tension, gas-solid surface tension and liquid-solid surface tension) in the horizontal direction on the triple phase line. We call the non-zero resultant force as unbalanced Young force (FYoung). The unbalanced Young force make the meltwater film dewet, producing the arm of force; with the unbalanced surface force which works as the acting force, a torque is generated. Profiting from the ultralow adhesion property of the superhydrophobic surface, the torque curls the edge of the meltwater film (Fig. 4(c)). On common surfaces, although the two 6 / 14


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unbalanced forces exist, the large surface adhesion counteracts the two forces and prevents the occurrence of the edge curling; therefore, the meltwater dewets only by shrinkage. 25, 39

Figure 4 Schematic diagram of edge curling phenomenon of meltwater film on superhydrophobic surfaces. (a) Original frost layer on superhydrophobic surfaces; (b) layered frost meltwater film (ice slurry in the upper layer and pure water in the lower layer) with unbalanced surface force (Fsur) in the interface between the ice slurry layer and the water layer, as well as unbalanced Young force (FYoung) in the triple phase line area; (c) a torque is generated and curls the edge of the meltwater film. Non-breaking of Chained Droplets Another interesting phenomenon is the non-breaking of chained droplets during the multi-meltwater evolution process on superhydrophobic surfaces. As Fig. 5 shows, although the liquid bridges between the chained droplets are quite thin, these droplets coalesce into large droplets instead of breaking up finally. See video S-2 in the supporting information for video display.

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Figure 5 Non-breaking of chained droplets on superhydrophobic surfaces. Although the liquid bridges between chained droplets are quite thin, these droplets coalesce into large droplets instead of breaking up (the arrows pointing to each other represent coalescence). See video S-2 in the supporting information for video display. To understand the mechanism of the non-breaking of chained droplets on superhydrophobic surfaces, a theoretical model was developed. Figure 6 is the schematic diagram of chained meltwater droplets from side-view and top-view. Where D is the droplet diameter; d is the liquid bridge diameter; L is the distance between the droplet center (note that L > D for chained droplets, otherwise the droplets touch each other and are certain to coalesce); W is base width of the droplets and W=D·sinθ0 where θ0 is the surface apparent contact angle; θa is advancing contact angle and θr is receding contact angle. Inspired by the pendant droplet method which measures the liquid surface tension by equating the droplet gravity with the component of the surface tension in the vertical direction, 40-41 we think that whether the chained droplets break up or not depends on the balance between the surface tension and the retentive force due to the contact angle hysteresis, given in Eq. (1). γπ d ↔ kγ W ( cos θr − cosθa )

(1)

where k is correction coefficient for the retentive force with k set to be 2 here. 42-45 Considering the critical case that the surface tension is equal to the retentive force, the critical ratio of the liquid bridge diameter to the droplet diameter was obtained as d 2 = sin θ0 ( cos θ r − cos θa ) D cri π

(2)

When the actual ratio is more than the critical ratio, the surface tension dominates and the chained 8 / 14


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droplets coalesce; otherwise the retentive force is dominant and the liquid bridge fractures with the chained droplets breaking up. Equation 2 could be an approximate criterion for the judgement of coalescence or breaking up of chained droplets. For the experimental superhydrophobic surface (θ0 = 160o, θa =162o and θr =158o) used here, the critical d/D is about 0.005. The critical d/D is so small that almost all actual value is more than it, which means the surface tension is always dominant compared to the retentive force, so the chained droplets will not break up on the superhydrophobic surface.

Figure 6 Schematic diagram of chained meltwater droplets from side-view and top-view. d is the liquid bridge diameter; D is the droplet diameter; L is the distance between the droplet center; W is base width of the droplets and W=D·sinθ0 where θ0 is apparent contact angle; θa is advancing contact angle and θr is receding contact angle. To further validate the effectiveness of Eq. (2), we conducted defrosting experiments not only on the superhydrophobic surface but also on three hydrophobic surfaces (HS1, HS2 and HS3, their detailed contact angle measurement results are shown in Fig. S-2 in the supporting information). The status (coalescence or breaking up) of chained droplets on these surfaces was observed with the d/D values recorded. Figure 7 shows the results. In Fig. 7, solid symbols represent coalescence of chained droplets, hollow symbols represent breaking up and short lines are critical d/D values by Eq. (2). As seen, on all the four surfaces, when the actual d/D value is more than the critical one, the chained droplets coalesce, otherwise they break up. Thus, Equation 2 works well as a criterion.

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Figure 7 Coalescence or breaking up of chained droplets on various surfaces. Y-coordinate (d/D) is ratio of the liquid bridge diameter to the droplet diameter and x-coordinate is surface. SHS is the superhydrophobic surface used in experiments; HS1, HS2 and HS3 are three hydrophobic surfaces (see their contact angle measurements in Fig. S-2 in the supporting information). Solid symbols represent coalescence in experiments, hollow symbols represent breaking up in experiments and short lines represent critical cases in theory. Conclusions In summary, this work investigated the meltwater evolution characteristics during defrosting on superhydrophobic surfaces when the original frost layer is not too thick (generally less than 1 mm). With some new phenomena reported, this work tried to address some unclear issues in previous researches. The meltwater film usually experiences two stages: first dewets into a slender meltwater column in a way of edge curling, and then shrinks into a spherical droplet. Profiting from the ultralow surface adhesion of the superhydrophobic surface, the edge curling is a distinct phenomenon driven by a torque produced by two unbalanced forces acting on the layered meltwater film (ice slurry in the upper layer and pure water in the lower layer). However, although the mechanism of edge curling is explained, we haven’t measured the effective surface tension of the ice slurry to give quantitative estimation of the unbalanced forces, which is worth doing in our future work. The non-breaking phenomenon of chained droplets during the multi-meltwater evolution process on superhydrophobic surfaces is also interesting. Whether the chained droplets break up or not depends on the balance between the surface tension and the retentive force due to the contact angle hysteresis. On the superhydrophobic surface, the contact angle hysteresis is quite small with negligible retentive force, so the surface tension always dominates and coalesces the chained droplets. An approximate criterion was then developed to judge the status (coalescence or breaking up) of chained droplets, and experimental data from various surfaces (also including hydrophobic surfaces) have validated the effectiveness of this criterion. We hope this work could deepen the understanding on the defrosting process on superhydrophobic surfaces and promote the anti-frosting/icing applications in the actual engineering projects. 10 / 14


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Supporting Information Video S-1 shows the evolution process of a meltwater film during defrosting on superhydrophobic surfaces. Video S-2 shows the non-breaking phenomenon of chained droplets during the multi-meltwater evolution process on superhydrophobic surfaces. Figure S-1 shows experimental images of multi-meltwater evolution process on superhydrophobic surfaces. Figure S-2 shows contact angle measurements of three hydrophobic surfaces (for comparison). Author information Corresponding Author: *E-mail: [email protected] ORCID: 0000-0001-7703-0038 Notes: The authors declare no competing financial interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No.51476084) and the National Natural Science Fund for Creative Research Groups (No. 51621062). References: 1. Rafati Nasr, M.; Fauchoux, M.; Besant, R. W.; Simonson, C. J. A review of frosting in air-to-air energy exchangers. Renewable Sustainable Energy Rev. 2014, 30, 538-554. 2. Wang, Z. Recent progress on ultrasonic de-icing technique used for wind power generation, high-voltage transmission line and aircraft. Energy Build. 2017, 140, 42-49. 3. Lynch, F. T.; Khodadoust, A. Effects of ice accretions on aircraft aerodynamics. Prog. Aerosp. Sci. 2001, 37, 669-767. 4. Nath, S.; Ahmadi, S. F.; Boreyko, J. B. A Review of Condensation Frosting. Nanoscale Microscale Thermophys. Eng. 2016, 21 (2), 81-101. 5. Sheng, W.; Liu, P.; Dang, C.; Liu, G. Review of restraint frost method on cold surface. Renewable Sustainable Energy Rev. 2017, 79, 806-813. 6. Villalpando, F.; Reggio, M.; Ilinca, A. Prediction of ice accretion and anti-icing heating power on wind turbine blades using standard commercial software. Energy 2016, 114, 1041-1052. 7. Fillion, R. M.; Riahi, A. R.; Edrisy, A. A review of icing prevention in photovoltaic devices by surface engineering. Renewable Sustainable Energy Rev. 2014, 32, 797-809. 8. Schutzius, T. M.; Jung, S.; Maitra, T.; Eberle, P.; Antonini, C.; Stamatopoulos, C.; Poulikakos, D. Physics of icing and rational design of surfaces with extraordinary icephobicity. Langmuir 2015, 31 (17), 4807-4821. 9. Kreder, M. J.; Alvarenga, J.; Kim, P.; Aizenberg, J. Design of anti-icing surfaces: smooth, textured or slippery? Nat. Rev. Mater. 2016, 1 (1), 15003. 10. Lv, J.; Song, Y.; Jiang, L.; Wang, J. Bio-inspired strategies for anti-icing. ACS Nano 2014, 8 (4), 11 / 14



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