超疏水状态的润湿转变与稳定性测试

Wetting Transition and Stability Testing of Superhydrophobic State

超疏水材料具有自清洁、防水、低粘附等特性,因此具有重要的应用价值.维持超疏水状态的稳定性,避免水侵入到材料表面微结构内部是实现这些特性的基础.本文在水下超疏水界面全反射的基础上,结合真空技术,提出了一种连续、直观的测试方法来测试超疏水状态的稳定性,并研究了Cassie-Wenzel润湿过渡行为及其临界压力.实验结果表明:对于典型的柱状微凸起结构,Cassie-Wenzel润湿转变过程可分为四个阶段:非润湿阶段、主要润湿阶段、强化润湿阶段和完全润湿阶段.主要润湿过程中的临界压力与理论值一致;强化润湿阶段需在较高的压力作用下进行,从而驱动固/液系统过渡到完全润湿阶段.与柱状结构相比,荷叶的乳突状微结构在润湿过程并不存在非润湿阶段,这是因为二者对外部压力的抵抗方式不同所致:柱状微结构通过增大柱子间悬挂液面的曲率来与外部压力建立平衡,而乳突状微结构则通过润湿过程中三相接触线密度的增加来强化毛细作用力,从而与外部压力建立平衡.

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.