Wetting dynamics study of underwater superhydrophobic surfaces through direct meniscus visualization

We study wetting of an air-filled micro-cavity on hydrophobic surface submerged in water by developing an optically clear sample that makes the location of the liquid-air meniscus inside the cavity visible. The plastron state, i.e., the state of the trapped air under water, is a central issue for the superhydrophobic surface research today because of its importance for many important applications, such as drag reduction. By continuously observing the meniscus on and inside a single trench during the wetting process, we obtain deterministic dynamics of the meniscus for the first time, as opposed to the probabilistic data in the recent studies. Our results confirm that the meniscus is in one of two states - pinned at the mouth of the trench or sliding on the sidewall of the trench, the latter leading to the fully-wetting (i.e., Wenzel) state. Furthermore, the results reveal that the dewetted (i.e., Cassie-Baxter) state can (or cannot) be indefinite if (or unless) the water is saturated with air and the hydrostatic pressure is low enough.