Ultrahydrophobic surfaces. Effects of topography length scales on wettability

We discuss dynamic hydrophobicity from the perspective of the force require d 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 seri es of silicon surfaces that were prepared by photolithography and hydrophob ized using silanization reagents. Hydrocarbon, siloxane, and fluorocarbon s urfaces were prepared. The surfaces contain posts of different sizes, shape s, and separations. Surfaces containing square posts with X-Y dimensions of 32 mu m and less exhibited ultrahydrophobic behavior with high advancing a nd receding water contact angles. Water droplets moved very easily on these surfaces and rolled off of slightly tilted surfaces. Contact angles were i ndependent of the post height from 20 to 140 mu m and independent of surfac e 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 conta ct angle increases to decreases in the contact length and increases in tort uosity of the three-phase contact line. The maximum length scale of roughne ss that imparts ultrahydrophobicity is similar to 32 mu m.