Demonstrating spray deposition of self-regulated nanorough layers for stable transparent superhydrophobic film coatings

To make large-scale onsite fabrication of transparent superhydrophobic films practical, the deposition technique should involve mechanisms that allow self-limitation of layer thickness within nanoscale level. Here, we propose a rapid and scalable deposition strategy that allows uniform placement of a nanorough layer, sufficient to generate superhydrophobic properties with enhanced optical clarity. The coating processes involved ionic spray-assisted layer-by-layer assembly to deposit a single layer of dual-scale rough silica particles with much improved kinetics, followed by rapid surface passivation using hot steam deposition of alkylsilane mixture in open air conditions. The whole processing can be finished within a few minutes with self-termination of film growth, producing well-controlled thicknesses within the range 200-400 nm. The fabricated films exhibited excellent superhydrophobic properties with water contact angle as high as 173 +/- 2 degrees and droplet roll-off angle as small as 2 +/- 1 degrees. Instead of being a hindrance, the nanorough layers increased the optical transmission in all cases, due to negligible scattering loss and the emergence of antireflection property. The fabrication of transparent superhydrophobic film was accomplished over broad ranges of coating conditions, indicating high tolerances in the input parameters. Additionally, the resulting films also showed acceptable stability in tests of droplet compression, water jet erosion, and degradation induced by ultraviolet irradiation, which is promising for outdoor implementations.