Molecular dynamics study on the wettability of a hydrophobic surface textured with nanoscale pillars.

Using molecular dynamics simulation, we studied the wetting properties of a surface textured with hydrophobic pillars, several nanometers in size. The drying transition of water confined between square or circular pillars was related to the Wenzel (WZ) to Cassie-Baxter (CB) transition of a water droplet deposited on periodic pillars. The inter-pillar spacing at which the drying occurs was compared to that predicted from the continuum theory. Such a comparison revealed that the line tension plays an important role in the drying behavior of the present nm-sized pillars. The water molecules near the pillar walls were layered and ordered in orientation. Our simulation showed a long-lived CB state which eventually turns into the WZ state. In this transition, water slowly penetrated down into the inter-pillar gap until it reached the half height of the pillar, and then quickly reached the base of the pillar.