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

摘要

The effect of physical roughness at multiple length scales on macro-, micro- and nanowetting behaviour of superhydrophobic surfaces was examined. Through the combination and modification of two existing techniques, Freeze Fracture Microscopy (FFM) and Synchrotron Small Angled X-ray Scattering (SAXS), a new complementary characterization system was developed to quantify wetting behaviour at the micro- and nanoscale.Tailored superhydrophobic surfaces were developed, which exhibited both fractal and non-fractal structures. Non-fractal surfaces were fabricated using photolithography to isotropically pattern micron-sized pillars followed by an additional nanoparticle coating. Fractal surfaces were fabricated using sol-gel chemistry to generate a hydrophobic, gelated nanoparticle system on a silicon substrate. The surfaces were immersed into aqueous fluids of various surface tensions and their wetting behaviour characterized in-situ. A wetting model was established by observing the progression of wetting through well-defined micro- and nanoroughness of non-fractal surfaces as a function of fluid surface tension. Applying this model to two fractal superhydrophobic surfaces, the origin of differing nanowetting behaviour despite identical macroscopic wettability was revealed to be the presence of pore structures, rather than roughness, at multiple length scales. Results revealed that, for non-fractal surfaces, the wetting progression is divided between preferential micro- or nano-wetting phenomena. Non-fractal superhydrophobic surfaces preferentially wet nanoscopically, whereas non-superhydrophobic surfaces wet microscopically. This suggests that the lowest free energy state can be fine-tuned by the varying pitch of the photolithographically patterned micro-roughness. Fractal superhydrophobic surfaces, on the other hand, undergo a more complex wetting progression and are governed by the aggregation behaviour of the silica nanoparticles. In-situ SAXS analysis of a nano-porous silica gel network revealed that a greater proportion of the structure is wettable compared to an engineered silica gel network consisting of intercalating micro-pores. This indicates that the distinct geometric features afforded by an engineered level of micro-pores within a fractal superhydrophobic surface provide a level of resilience against wetting.