Wetting Transition of a Cylindrical Cavity Engraved on a Hydrophobic Surface

This study theoretically examines the wetting of a cylindrical cavity engraved on a hydrophobic surface, in the context of the Cassie–Baxter-to-Wenzel transition of a water drop resting on such a surface. The stable, metastable, and transition states and their free energies are identified by constructing the free-energy profile of the wetting process. Wetting starts with a liquid–vapor interface pinned at the top edge of the cavity and proceeds with a symmetrically depinned interface. The liquid–vapor interface later becomes annular upon its touching of the bottom of the cavity and finally asymmetric before the cavity is fully wetted by the liquid. This study examines the effects of the cavity geometry and the pressure of the liquid on the wetting and dewetting transitions

Monte Carlo Study on the Wetting Behavior of a Surface Texturized with Domed Pillars

A lattice gas Monte Carlo simulation was performed to examine the wetting properties of a surface texturized with nanometer-sized, dome-shaped pillars. The vapor and liquid phases of the gap between the pillars were related to the Wenzel and Cassie–Baxter states of a macroscopic water droplet resting on top of the pillars. We studied the effects of the pillar size by systematically varying its height from 6 to 53 nm for a fixed ratio of the height to its width. With increasing interpillar spacing or pressure, the liquid on top of the domed pillars penetrated smoothly down into the gap between the pillars. This wetting transition contrasts with that observed for the gap between rectangular or cylindrical pillars, where a liquid abruptly fills in the interpillar gap at a critical interpillar spacing or pressure. The gap between the domed pillars was more susceptible to the intrusion of the bulk liquid on top of the pillars, due to the open geometry of the gap between the domed pillars. Also, the liquid penetrating into the gap between the domed pillars was locally more fluctuating in density and compressible than that penetrating into the gap between square or cylindrical pillars. This enhanced density fluctuation however was local and did not propagate into the bulk liquid sitting on top of the pillars. Simple analytic expressions of the critical spacing and pressure at which the wetting transition occurs for the domed pillars were derived using continuum theory. These continuum results agreed reasonably well with the present molecular simulations, even for pillars as small as a few nanometers in width