Hierarchical surfaces: an in situ investigation into nano and micro scale wettability

Two scales of roughness are imparted onto silicon surfaces by isotropically patterning micron sized pillars using photolithography followed by an additional nanoparticle coating. Contact angles of the patterned surfaces were observed to increase with the addition of the nanoparticle coating, several of which, exhibited superhydrophobic characteristics. Freeze fracture atomic force microscopy and in situ synchrotron SAXS were used to investigate the micro- and nano-wettability of these surfaces using aqueous liquids of varying surface tension. The results revealed that scaling different roughness morphologies result in unique wetting characteristics. It indicated that surfaces with micro, nano or dual scale roughness induced channels for the wetting liquid as per capillary action. With the reduction of liquid surface tension, nano-wetting behaviour differed between superhydrophobic and non-superhydrophobic dual-scale roughness surfaces. Micro-wetting behaviour, however, remained consistent. This suggests that micro- and nano-wetting are mutually exclusive, and that the order in which they occur is ultimately governed by the energy expenditure of the entire system.