Wetting Transition and Stability Testing of Superhydrophobic State

Superhydrophobic surfaces exhibit self-cleaning, water-repellency and anti-sticking properties, and thus have potential applications in various fields. Maintaining the stability of superhydrophobicity and avoiding the intrusion of water are essential preconditions for realization of these properties. Based on the total reflection of underwater superhydrophobic interface and vacuum technique, we propose a continuous and visual method for investigating the wetting behavior and critical pressure of Cassie-Wenzel transition. The results indicate that, for a typical surface covered by asperities, the wetting transition has four stages; non-wetting stage, primary wetting stage, enhanced wetting stage, and complete wetting stage. The critical pressure during the primary wetting stage agrees with the theoretical one. The enhanced wetting stage takes place at a relatively high pressure, which drives the solid/liquid system into the complete wetting stage. In comparison with columnar microstructures, the lotus leaf does not exhibit the non-wetting stage during the wetting transition. This difference lies in their resistance mechanisms; columnar microstructures adapt to external pressure by increasing the curvature of the meniscus that hangs between pillars, while papillary microstructures adapt to external pressure by enhancing the capillary force via increased density of three-phase contact line during the wetting process.