Influence of Roughness on a Transparent Superhydrophobic Coating

A novel fabrication process is presented using monodisperse PMMA latex particles to facilitate controlled microvoid formation. This results in hierarchically rough surfaces exhibiting similar to 90% optical transmission while retaining water contact angle (theta) of 170 degrees. Synchrotron small angle X-ray scattering, AFM roughness measurements, and theoretical modeling suggests that a surface morphology with fractal dimension of similar to 2.6 and R-a < 400 nm allows for the optimum coupling of roughness-induced superhydrophobicity and optical transparency. Interestingly, surfaces of vastly different roughness (R-rms) exhibited similar water contact angles, highlighting a limitation of traditional AFM roughness measurements in quantifying multiscale rough surfaces. An alternate method considering fractal dimension is presented as a more complete quantifier of hierarchical surface morphology in relation to surface wetting behavior.