Wetting of Surfaces Made of Hydrophobic Cavities

Templated electrodeposition through a close packed, monolayer array of 3 mu m polystyrene spheres followed by removal of the template by dissolution in an organic solvent was used to fabricate sphere segment void (SSV) surfaces in gold with heights up to 1.5 mu m. These surfaces were made hydrophobic by treating with 1-dodecanethiol. Contact angle measurements show that the wetting behavior of these surfaces change significantly with film thickness. The apparent advancing contact angle increases from 110 degrees for the flat 1-dodecanethiol-coated gold surface to 150 degrees for the film with a close-packed array of hemispherical cavities, in good agreement with the behavior predicted by the simple Cassie-Baxter equation. In contrast, the apparent receding angles have significantly smaller values in all cases, and water droplets are strongly pinned at the surface. Thus, these surfaces demonstrate "rose petal" behavior, in which a large apparent advancing contact angle, typical of a superhydrophobic surface, is accompanied by significant contact angle hysteresis. Observation of the shapes of drops on the surface during evaporation-driven recession shows that the drops adopt a dodecagonal shape, in which the drop perimeter is selectively pinned along the < 10 > and < 11 > directions on the hexagonally close-packed surface.