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

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.

💡 Research Summary

The paper addresses a critical limitation of superhydrophobic surfaces: loss of water‑repellent performance under high humidity and low temperature due to condensation. When condensate droplets fill the micro‑ and nano‑scale cavities, the trapped air layer collapses and the wetting state switches from Cassie‑Baxter to the energetically favored Wenzel state, dramatically reducing the apparent contact angle. To recover superhydrophobicity, the authors propose a simple yet effective “freezing‑melting” strategy.

Two types of superhydrophobic substrates were fabricated on silicon wafers: (i) a single‑scale nano‑structured surface (SN) created by spray‑coating a silica‑particle slurry, and (ii) a hierarchical micro‑nano‑structured surface (HMN) formed by coating PDMS and then depositing silica particles to generate micro‑scale pillars topped with nano‑roughness. Both surfaces exhibit static contact angles above 150° in their pristine Cassie‑Baxter state.

The experimental setup consists of a copper plate whose temperature is precisely controlled by a semiconductor temperature‑control system, enclosed within an acrylic chamber maintained at 60 % relative humidity. Condensation experiments were performed at substrate temperatures of 8 °C and 2 °C. High‑resolution CMOS imaging captured the nucleation, growth, and coalescence of condensate droplets, while a high‑speed camera recorded droplet impact dynamics. Results show that the SN surface, with its narrow, uniform nanogaps, quickly becomes inundated with condensate; the apparent contact angle drops from 151° to 116° at 2 °C, indicating a complete wetting transition. The HMN surface, possessing larger micro‑cavities, retains some air pockets even after prolonged condensation, and its contact angle reduction is less severe.

The authors analyze the wetting transition thermodynamically. The free‑energy difference between Cassie‑Baxter (non‑wetting) and Wenzel (wetting) states (ΔG₁) favors the Wenzel state when condensate fills the texture, and an additional energy barrier (ΔG₂) must be overcome for the reverse transition. In practice, condensation supplies the necessary energy by displacing air and increasing solid‑liquid contact area.

The core of the study is the freezing‑melting cycle. Step I: the substrate temperature is rapidly lowered to –25 °C, freezing the droplet. Ice nucleation is asymmetric and generates volumetric expansion, which physically pushes the previously condensed liquid out of the texture and re‑creates air pockets. Step II: the substrate is heated to 30 °C, melting the ice while preserving the newly formed air layer. After melting, the static contact angle recovers to 166°, confirming a transition back to the Cassie‑Baxter state. Importantly, SEM inspection before and after the cycle shows no damage to the surface morphology, indicating that the process is non‑destructive.

Temperature transitions are achieved within 15 seconds, thanks to the fast response of the semiconductor controller, and the entire protocol requires only temperature control—no electric fields, magnetic fluids, or complex mechanical actuation. This simplicity makes the method readily integrable into existing systems.

Key insights include: (1) condensation‑induced loss of superhydrophobicity can be reversed by exploiting the volumetric expansion of ice during freezing; (2) hierarchical micro‑nano textures provide greater resistance to condensation‑driven wetting than single‑scale nano textures; (3) rapid temperature cycling offers a practical, low‑energy route to restore the Cassie‑Baxter state without altering the surface.

The findings have broad implications for applications where superhydrophobic coatings are exposed to cold, humid environments, such as heat exchangers, aircraft surfaces, and electronic cooling devices. By incorporating a brief freezing‑melting step, designers can maintain low drag, anti‑icing, and self‑cleaning performance over extended service periods. The work thus advances the fundamental understanding of phase‑change‑mediated wetting dynamics and provides a viable engineering solution for preserving superhydrophobic functionality.