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Dynamic Melting of Freezing Droplets on Ultraslippery Superhydrophobic Surfaces Fuqiang Chu, Xiaomin Wu,* and Lingli Wang Department of Thermal Engineering, Beijing Key Laboratory for CO2 Utilization and Reduction Technology, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: Condensed droplet freezing and freezing droplet melting phenomena on the prepared ultraslippery superhydrophobic surface were observed and discussed in this study. Although the freezing delay performance of the surface is common, the melting of the freezing droplets on the surface is quite interesting. Three self-propelled movements of the melting droplets (ice− water mixture) were found including the droplet rotating, the droplet jumping, and the droplet sliding. The melting droplet rotating, which means that the melting droplet rotates spontaneously on the superhydrophobic surface like a spinning top, is first reported in this study and may have some potential applications in various engineering fields. The melting droplet jumping and sliding are similar to those occurring during condensation but have larger size scale and motion scale, as the melting droplets have extra-large specific surface area with much more surface energy available. These self-propelled movements make all the melting droplets on the superhydrophobic surface dynamic, easily removed, which may be promising for the anti-icing/frosting applications. KEYWORDS: freezing, dynamic melting, superhydrophobic surface, self-propelled movement, droplet rotating phobicity.19,20 At this time, the deicing/frosting is still essential, which indicates that the deicing/frosting characteristics of the superhydrophobic surface need to be understood. Boreyko et al. conducted defrosting experiments on a horizontal superhydrophobic surface with their observations of the defrosting mainly including the partial melting and the spontaneous dewetting (unstable slush film composed of meltwater and frost crystal dewetted into mobile slush ball).21 Chen et al. observed that the bulk frost was fractured into many irregular pieces upon melting on the hydrophobic surface, while the nanograssed superhydrophobic surface exhibited a relative low fracture density and the melting frost became a large spherical water droplet at the culmination of defrosting.22 Wang et al. observed that an irregular frost crystal (sectional dimensions 50 μm × 60 μm) completely jumped out from the substrate with self-rotation during the defrosting process.23 However, the motion details and the motion mechanism of the melting droplet have not been shown, and no one else has reported that. Inspired by the distinct motion of condensed droplets, we thought intuitively that the melting droplet motion on the superhydrophobic surface may be also attractive, and the research on it is instructive for the anti-icing/frosting applications. Thus, superhydrophobic surfaces were fabricated with related experiments conducted to observe the droplet
1. INTRODUCTION Icing/frosting problems, which exist widely in nature and human industry, cause negative effects for both human safety and the industrial production.1,2 Many efforts have been made to solve the icing/frosting problems. In recent years, as the development of the surface engineering, superhydrophobic surfaces with great water repellency have been used to restrain the ice/frost formation.3−7 Due to its large water contact angle, the superhydrophobic surface has several advantages to reduce the ice/frost growth, such as large nucleation energy barrier for ice nucleation, small ice−surface contact area for heat transfer, and weak ice/frost adhesion.6−8 Recently, Boreyko and Chen first reported the out-of-plane jumping motion of the condensed droplets on the superhydrophobic surface.9 The out-of-plane droplet jumping, triggered by two or multidroplet coalescence and driven by the released surface energy, is a kind of self-propelled droplet motion that does not depend on any external forces.9 Some researchers continued to study the droplet jumping phenomenon on various superhydrophobic surfaces with its motion mechanism analyzed, and another selfpropelled droplet motion called the droplet sweeping was also observed and characterized.10−15 These self-propelled droplet movements were thought to be a new way to restrain ice/frost, as the subcooled droplets may be able to be repeatedly removed from the surface before ice nucleation occurs.16−18 However, when considering the harsh environmental conditions such as lower temperatures and higher humidity, even the superhydrophobic surface failed to retain ice© XXXX American Chemical Society
Received: December 30, 2016 Accepted: February 21, 2017 Published: February 21, 2017 A
DOI: 10.1021/acsami.6b16803 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
freezing droplets began to melt naturally in the surrounding air (because of the heating effect of the surrounding air, the surface temperature was rising quickly at stage 3, as shown in the Supporting Information S1).
freezing and the freezing droplet melting phenomena in this study.
2. EXPERIMENTAL SECTION Surface Preparation. An Al-based superhydrophobic surface was prepared as the experimental surface using the chemical etching− deposition method.24 The experimental surface preparation mainly included two steps. First, the polished aluminum surface was immersed into the hydrochloric acid and copper nitrate mixed solution (0.1 mol/ L HCl and 0.1 mol/L Cu(NO3)2) for surface structure construction; then the aluminum surface was immersed into a fluoroalkylsilane (1H,1H,2H,2H-perfluorodecyltriethoxysilane) solution (1 wt %) to obtain low surface energy.24 As the SEM (scanning electron microscope) images show in Figure 1, the experimental surface is
3. RESULTS AND DISCUSSION Condensed Droplets Freezing. During stage 1 of the experiment, when the experimental surface temperature was about −5.0 °C, the droplet condensation occurred and the antiicing phenomenon was demonstrated with no droplet freezing found. The liquid droplets on the superhydrophobic surface maintain very high mobility with the coalescence between them easily triggering a self-propelled movement, such as the droplet jumping and the droplet sweeping.9,15 Because of their wide application prospects in engineering, these self-propelled droplet movements have been reported and analyzed in a large quantity of literature,9−15 and we will not review them here. At stage 2 of the experiment, the experimental surface temperature was reduced to −12.0 °C. Although the freezing time was more or less delayed, the condensed droplets began to freeze and the whole surface was covered by freezing droplets finally. Figure 2a shows the condensed droplet freezing and the
Figure 1. SEM images of the experimental superhydrophobic surface. The experimental surface is covered by microstructures with the characteristic sizes of 5−20 μm, with these microstructures being aggregations of irregular nanoscale particles. covered by microstructures with the characteristic sizes of 5−20 μm with these microstructures being aggregations of irregular nanoscale particles.24 The hierarchical surface structures are the key points for the surface to maintain its superhydrophobicity, including large contact angle and small rolling angle. The measured contact angle of a 2 μL deionized water droplet by a contact angle goniometer (JC2000C1, China) is 164.0° ± 0.5°. The rolling angle of the experimental surface is measured to be 1.0° ± 0.5°, which indicates its ultraslippery feature such that we cannot even place the testing droplet smoothly on the surface. Experimental System and Conditions. The experimental system used in this research was the same as that in refs 24 and 35. The experimental surfaces were horizontally placed on the cold side of a thermoelectric cooler with the experiments performed in a closed laboratory. During the experiments, according to different surface temperature, three experimental stages were divided. The experimental conditions and duration times for the three stages are shown in Table 1 (see variations of the experimental conditions during the experiments in the Supporting Information S1). Droplet condensation occurred on the superhydrophobic surface at stage 1 (anti-icing phenomenon has been demonstrated on the superhydrophobic surface with a not too low surface temperature such as − 5 °C 18). Then, when the surface temperature was reduced below − 10 °C, the condensed droplets began to freeze after a few minutes of delay at stage 2. At stage 3, when the power of the thermoelectric cooler was shut off,
Figure 2. (a) Condensed droplet freezing and the freezing wave propagation on the superhydrophobic surface. (b) Statistical data of the freezing delay time and the freezing finish time on the superhydrophobic surfaces. (c) Ice droplet morphologies at the end of stage 2, where frost crystals grow densely on the ice droplet surface.
freezing wave propagation on the experimental superhydrophobic surface. The early freezing droplets are located randomly on the surface with the freezing waves propagating from different freezing droplets, meeting each other, until all the droplets on the whole surface freeze completely. The freezing delay time (the time when the first droplet freezes at stage 2) and the freezing finish time (the time from the first droplet freezing to the last droplet freezing) are counted from seven tests in Figure 2b. The results show that the average freezing delay time is about 1.5 min and the average freezing finish time is about 6 min. The freezing delay performance may not be so good as those in the literature,4,5,18,25 and the reason is mainly attributed to the experimental conditions with high air humidity. Previous studies have shown that under high air humidity conditions, heterogeneous nucleation from the surface takes place, while the homogeneous nucleation from the gas−water droplet interface is the favored mode under low air humidity conditions.26 In our experiment, the air humidity is quite high with the heterogeneous nucleation preferentially occurring. The heterogeneous nucleation is greatly susceptible to the surface characteristics with some surface defects or surface impurities easily resulting in a nucleation.27 Once a heterogeneous nucleation takes place, it initiates freezing in a droplet immediately. Therefore, the freezing delay time is shortened.
Table 1. Experimental Conditions stage
conditions
duration (min)
stage 1
Tsur = −5.0 ± 0.1 °C Tair = 18.8 ± 0.5 °C RH = 85.0 ± 5.0% Tsur = −12.0 ± 0.1 °C Tair = 18.8 ± 0.5 °C RH = 85.0 ± 5.0% Tsur = −12.0 to 18.8 °C Tair = 18.8 ± 0.5 °C RH = 85.0 ± 5.0%
20
stage 2
stage 3
15
