Contact Angle Studies on Porous Silicon: Evidence for Heterogeneous Wetting and Implications of Oxidation

A study of wetting was carried out on porous silicon films with pore diameters spanning three orders of magnitude. Water contact angle measurements on adjoining porous and nonporous regions yielded Wenzel roughness ratios that were either unphysical (less than unity) or unrealistically low when compared to those expected from specific surface area considerations. Moreover, results obtained from a sample consisting of a microporous film on a macroporous layer gave contact angles that were very similar to those found on films consisting of only a single microporous layer, contrary to what would be expected for complete filling of the pores by liquid. Values for wetted surface fractions calculated from the Cassie-Baxter model are unreasonably high for micro- , meso- and oxidized macroporous films considering their porosities, while relatively oxide-free macroporous films give wetted surface fractions in accord with those expected based on film porosity. Collectively, these results show that the predominant mode of wetting on these films is heterogeneous.