Hydrophobicity of model surfaces with loosely packed polystyrene spheres after plasma etching.

Polystyrene (PS) sphere films with loosely packed arrays were prepared by plasma etching of closely packed PS sphere arrays. The size of PS spheres can be efficiently reduced with plasma etching, and surface topography can be manipulated by controlling the initial PS sphere size and the time of plasma exposure. These surfaces with loosely packed arrays provide a well-characterized model system for studying water repellency behavior. It was found that the surface hydrophobicity could be systematically tailored due to the well-defined and controlled surface topography. Sphere size and the interparticle distance between two adjacent spheres are critical factors in determining the water repellency behavior of the surface. A model based on the Cassie theory was proposed to elucidate the effect of surface topography on hydrophobicity, and the predicted contact angles agree well with the experimental results.