Superhydrophobicity and Optical Transparency in Thin Films: Criteria for Coexistence

The balance between roughness-induced superhydrophobicity and its impact on optical transparency was examined using 1-μm thick “seeded” sol–gel coatings. Conventional one-pot synthesis with fumed silica nanoparticles (12–40 nm) cross-linked into a three-dimensional porous network generated controlled hierarchically rough surfaces with apparent contact and sliding angles of θ CA > 150° and θ S < 10°, respectively. The transparency of such films in the visible light spectrum varied from 82% to 100% and was largely dependent on the domain size of silica nanoparticle clusters, both within the gel and reflected in the level of surface roughness. This was confirmed using both transmission small angle X-ray scattering and Fourier transform analysis of SEM images of superhydrophobic films obtained through cross-sectional milling. Differences in nanoparticles cluster size correlated with a scattering model where cluster size above 150 nm suggests bulk scattering is dominant, with surface scattering not contributing until the surface feature is greater than 400 nm. Concurrently, the surface topology of such coatings requires an enrichment of nanoscaled roughness superimposing over secondary submicrometer surface roughness, with an overall mean surface roughness >30 nm and smallest feature fractal dimension ≥3, to produce both superhydrophobicity and visible transparency.