Development of superhydrophobic coatings

© 2011 Dr. Kwun Lun ChoUltrarough surfaces with variable architecture were synthesised and examined using a combination of atomic force microscopy, synchrotron small angle X-ray scattering and contact angle goniometry. Using silica nanoparticle sol-gel technology, hierarchical superhydrophobic surfaces with water contact angles > 150° and hysteresis < 10° were fabricated. The addition and subsequent removal of 400 nm latex particles facilitated in carefully controlled, micro-pore formation, resulting in, amongst other things, a transparent superhydrophobic thin film. Varying nanoparticle size from 7 - 40 nm within the coating, pseudo-fractal dimension measured using SAXS (Small Angle X-ray Scattering) was shown to be critical in optimising superhydrophobicity. Combined with characterisation using Mie light scattering models, surfaces with RMS roughness 2.6 was found to exhibit both superhydrophobicity and optical transparency. The randomly aggregated sol-gel coating presenting an ideal platform to study the wetting of hierarchical rough surfaces similar to those in nature. Using in-situ SAXS measurements, combined with contact angle goniometry, the liquid/solid interface of a superhydrophobic surface at various stages of wetting was directly probed. For the first time, an intermediate wetting state whereby a surface is wetted at the macroscale but unwetted at the nanoscale, was observed in situ. The behaviour of superhydrophobic surfaces in practical applications, icing and marine biofouling, were examined. Superhydrophobic surfaces, due to its de-wetting properties, drastically reduced the adhesion strength of ice formed from liquid water. The coating further acted as an heat insulator, inhibiting frost formation from cold humid air. In marine anti-fouling applications, hierarchical superhydrophobic surfaces show considerable promise in resisting the initial attachment of marine organisms. However, field biofouling experiments suggest that micro-roughness and surface chemistry are dominant factors in antifouling