Investigation of optically transparent non-wetting coatings

© 2019 Chang WangSurfaces that exhibit extreme non-wettability are the result of the combination of surface chemistry and a high degree of roughness. Typically, such materials are visibly opaque, predominantly due to the enhanced light-scattering from the rough interfaces. In the present study, particle-based coatings with carefully controlled surface and bulk roughness parameters are investigated in multiple length scales to identify conditions under which extreme surface-phobicity and visible transparency can coexist. Visibly transparent micron-thick fumed silica nanoparticle (7–40nm) seeded sol-gel coatings were fabricated. Nano-scale surface and bulk features with varying degrees of water-repellency (hydrophobicity) were correlated with visible optical transparency. Furthermore, alternative tuneable hollow silica spheres in the sub- micron scale range were utilised to create a finely detailed range of roughness, which simultaneously changes both the refractive index through lowering relative optical permittivity and film density, and the surface wettability. Surface morphology and roughness of individual samples were examined in detail using atomic force microscopy, non-contact optical profilometry, scanning electron microscopy and synchrotron-based small-angle X-ray scattering (SAXS) in transmission mode. The latter was employed to investigate the nano-scaled common length scale (repeating) features in the coating. Coatings consisted of hierarchically ordered structures similar to fractals, with nano-scale (12–30nm) features superimposed on top of larger (200–400nm) sub-micron features displaying coexistence of optical transparency and water repellency. Extended surface studies were achieved by sputtering a thin (8–15nm) layer of sputter-coated Cr metal, which enhanced the surface X-ray scattering. Coatings fabricated using colloidal fractals with enrichment of both nano (20–30nm) and sub-micron (50–270nm) length scale on its surface structure displayed an enhancement to the roughness-induced water-repellency behaviour. Analysis of the bulk features alone was performed through the cross-sectional imaging under dual- beam FIB/SEM. Visible optical transparency in the film required nanoparticle cluster size to be between 130–180nm in length, to minimise the scattering of visible light. Alternatively, through the manipulation of the size of hollow core and silica shell thickness, the optical transparency at specific wavelengths of visible light correlated to the size of hollow silica spheres is enhanced without the loss of roughness. Hollow silica spheres within the sub-micron length scale (300–700nm) provide an alternative system for fabricating roughness-induced optically transparent non-wettable coating