Nanodesign of superhydrophobic surfaces
Abstract
When deposited on a hydrophobic rough surface, a small water drop looks like a small pearl. In this study we investigate the conditions to observe such a phenomenon when the roughness of the substrate, of fractal nature, consists of nanofeatures obtained by the ion track etching technique. The surface roughness was characterized by two parameters: the fractal dimension Df and the aspect ratio AR. Both were extracted from the analysis of atomic force microscopy images and were directly correlated with the etching time. Df is found in the interval of 2.46–2.69, while AR varies from 0.5 to ~15. We show that for the highest Df values the water contact angles on these surfaces can exceed 150°, while the hysteresis—the difference between the advancing and receding contact angles—is reduced to 4–5°. Superhydrophobic surfaces can thus be obtained and this property seems well controlled by the fractal dimension, which is itself controlled by the ion track etching process. Furthermore, the experimental results are well described by the wetting theory, and it is shown that superhydrophobicity of these surfaces results from a single-scale roughness of nanometric size.