Diffusion-driven directional wetting transitions

We report on directional wetting transitions in gas-entrapping microtextured surfaces (GEMS) submerged in water. GEMS comprise SiO₂/Si surfaces adorned with arrays of microscale doubly reentrant cavities (DRCs) that have been demonstrated to entrap air underwater regardless of their chemical make-up. Here, we investigated time-dependent wetting transitions of water into DRCs as a function of hydrostatic pressure and compared them with simple cylindrical cavities. Experiments revealed that the cavities in the center sustained air-filled Cassie-states for significantly longer times than the boundary cavities, even though they had identical dimensions, surface chemistry, and liquid entry pressure. In fact, cavity filling always advanced boundary inwards, and the lifetimes of inner side cavities could be tuned to vary dramatically based on cavity size, pitch, surface density, and water column height. We will explain these findings based on our analytical calculations and computational simulations. Insights from this work will aid in creating coating-free surfaces capable of retaining air underwater over longer durations.