Biologically inspired surfaces: Broadening the scope of roughness

Superhydrophobicity can be used for many applications that require non-adhesive and water-repellent surfaces. A successful design of superhydrophobic surfaces requires a correct assessment of the surface roughness effect on wetting. Roughness is an important property in surface mechanics, physics, chemistry, and biology, and it is critical for many tribological applications. Roughness can be defined in different ways, and the definition should be adequate to the problem under investigation. Our recent studies of biological and biomimetic superhydrophobic surfaces show that traditional roughness parameters, such as the root-mean-square, correlation length, or fractal dimension, are not always appropriate for the analysis of wetting. This is, in particular, due to the hierarchical nature of wetting mechanisms and interfaces. We discuss the effect of roughness on wetting mechanisms and relevant roughness parameters and ways to broaden the concept and scope of surface roughness.