Systematic Control of Hydrophobic and Superhydrophobic Properties Using Double-Rough Structures Based on Mixtures of Metal Oxide Nanoparticles

Double-rough surfaces were prepared by using mixtures of metal oxide nanoparticles with diameters on two different length scales. The degree of double-roughness of these surfaces was controlled using defined ratios of larger (approximate to 300 nm) and smaller (approximate to 30 nm) particles. If the preparation method was chosen such that the smaller particles decorate the larger ones, a roughness on two length scales was induced and an increase of contact angles from 130 degrees to 160 degrees with increasing double-roughness was observed. At highest double-roughness, a transition from the Wenzel to Cassie regime occurred. The results were quantitatively modeled on the basis of a modified Wenzel equation taking into account the two hierarchies of roughness.