Superhydrophobicity and liquid repellency of solutions on polypropylene.

The sliding of drops of aqueous solutions of organic liquids over a superhydrophobic polypropylene (SH-PP) surface has been studied experimentally. The multi-scale roughness of this surface is intrinsically inhomogeneous. Careful analysis of the wettability of each solution enables us to establish the statistics of the advancing and receding contact angles. We consider a threshold value of the receding static contact angle, above which drop sliding and rebound are facilitated as the criterion for superhydrophobicity. The percentage of receding contact angles greater than this threshold is then used as a practical index of superhydrophobicity (SHI). The variation of the SHI is compared with surface tension of the solution and various wetting parameters. A linear correlation is found between the SHI and the work of wetting defined by γ(LG)cosθ(0) where γ(LG) is the surface tension of the solution and θ(0) is the static contact angle of the solution over the corresponding smooth surface. Such a correlation can be used as a predictive tool of the superhydrophobicity of a given surface with various liquids.