Ultrahydrophobic surfaces.: Effects of topography length scales on wettability

We discuss dynamic hydrophobicity from the perspective of the force required to move a water droplet on a surface and argue that the structure of the three-phase contact line is important. We studied the wettability of a series of silicon surfaces that were prepared by photolithography and hydrophobized using silanization reagents. Hydrocarbon, siloxane, and fluorocarbon surfaces were prepared. The surfaces contain posts of different sizes, shapes, and separations. Surfaces containing square posts with X-Y dimensions of 32 mu m and less exhibited ultrahydrophobic behavior with high advancing and receding water contact angles. Water droplets moved very easily on these surfaces and rolled off of slightly tilted surfaces. Contact angles were independent of the post height from 20 to 140 mu m and independent of surface chemistry. Water droplets were pinned on surfaces containing square posts with larger dimensions. Increasing the distance between posts and changing the shape of the posts from square to staggered rhombus, star, or indented square caused increases in receding contact angles. We ascribe these contact angle increases to decreases in the contact length and increases in tortuosity of the three-phase contact line. The maximum length scale of roughness that imparts ultrahydrophobicity is similar to 32 mu m.