Freezing-Melting Mediated Dewetting Transition for Droplets on Superhydrophobic Surfaces with Condensation

TL;DR

Condensation in humid environments can drive a wetting transition from Cassie-Baxter to Wenzel on superhydrophobic surfaces, compromising repellency. The authors propose a freezing–melting strategy, where the droplet is frozen by cooling and then melted by heating to induce a dewetting transition from Wenzel back to Cassie-Baxter. They compare a single-scale nano-structured surface (SN) with a hierarchical micro-nano-structured surface (HMN) to reveal structure-strength effects on the transition, using energy considerations with $\Delta G_1$ and $\Delta G_2$ to explain reversibility. The results show that the SN surface can achieve a robust dewetting during melting while HMN is more prone to remaining wetted or damaged; this mechanism informs design of durable, temperature-controllable superhydrophobic surfaces for cold, humid environments.

Abstract

The water-repellence properties of superhydrophobic surfaces make them promising for many applications. However, in some extreme environments, such as high humidities and low temperatures, condensation on the surface is inevitable, which induces the loss of surface superhydrophobicity. In this study, we propose a freezing-melting strategy to achieve the dewetting transition from the Wenzel state to the Cassie-Baxter state. It requires freezing the droplet by reducing the substrate temperature and then melting the droplet by heating the substrate. The condensation-induced wetting transition from the Cassie-Baxter state to the Wenzel state is analyzed first. Two kinds of superhydrophobic surfaces, i.e., single-scale nano-structured superhydrophobic surface and hierarchical-scale micro-nano-structured superhydrophobic surface, are compared and their effects on the static contact states and impact processes of droplets are analyzed. The mechanism for the dewetting transition is analyzed by exploring the differences in the micro/nano-structures of the surfaces and it is attributed to the unique structure and strength of the superhydrophobic surface. These findings will enrich our understanding of the droplet-surface interaction involving phase changes and have great application prospects for the design of superhydrophobic surfaces.

Figures (13)

• Figure 1: (a) Schematic diagram of the experimental setup. (b, c) SEM images of the two superhydrophobic surfaces: (b) single-scale nano-structured (SN) superhydrophobic surface; (c) hierarchical-scale micro-nano-structured (HMN) superhydrophobic surface.
• Figure 2: Wetting and dewetting transitions of a droplet on a superhydrophobic surface under the influence of temperature (Movie 1). Panels (a) and (b) show that the droplet undergoes a wetting transition due to the condensation on the superhydrophobic surface at a low temperature. Panels (c) and (d) show the freezing-melting strategy to recover the superhydrophobicity, which includes two steps: decreasing the surface temperature to freeze the droplet (i.e., Step I), and then increasing the surface temperature to melt the droplet (i.e., Step II). The heating temperature is 30 $^\circ$C, and the droplet volume is 37.1 µl.
• Figure 3: Condensation on (a) SN superhydrophobic surface and (b) HMN superhydrophobic surface: (a-c, e-g) image sequences showing the condensation condition over time at the substrate temperature of 8 $^\circ$C; (d, h) images showing the condensation state at the substrate temperature of 2 $^\circ$C and the cooling time of 180 s, for comparison with that in panels (b) and (f). To avoid the influence of observation position on the experimental results, the field of view of the camera is constrained within a region ($5 \times 5$ cm$^2$) at the center of the superhydrophobic surface ($40 \times 40$ cm$^2$).
• Figure 4: Static contact angles on the SN and HMN superhydrophobic surfaces under different experimental conditions, and wetting transitions for droplets on the superhydrophobic surface. (a) Variation of the static contact angle over the cooling time at the substrate temperature of 2 $^\circ$C. (b) Variation of the static contact angle against the substrate temperature for the SN superhydrophobic surface at the cooling time of 180 s. (c) Schematic illustration of wetting transition due to condensate droplets within the micro/nano-structures of the surface. (d) Variation of the free angle for different contact states of droplets on a superhydrophobic surface.
• Figure 5: Image sequences showing the impact processes of droplets on the SN and HMN superhydrophobic surfaces with different degrees of condensation: (a) for the SN substrate at 30 $^\circ$C (i.e., without cooling, Movie 2); (b) for the SN substrate with a temperature of 2 $^\circ$C for condensation (Movie 3); (c) for the SN substrate with a temperature of -25 $^\circ$C for condensation and freezing (Movie 4); (d) for the HMN substrate with a temperature of 2 $^\circ$C for condensation.