Producing super-hydrophobic surfaces with nano-silica spheres

Polycrystalline alumina substrates were dip-coated in dilute suspensions formed with dispersed, nano-silica particles. The fractional surface coverage of the alumina substrate was varied between approximate to 0.05 to approximate to 0.4 by changing concentration of particles in the silica slurry. After a heat treatment to partially sinter the particles to the surface, the surface was made hydrophobic by a reaction with a solution containing, fluorosilane molecules. Wetting measurements showed that the contact angle between the surface and water droplets increased with decreasing area coverage of nano-silica spheres, consistent with a previous theory, modified here to include the pressure of the water droplet. The modification of the theory predicts that the super-hydrophobic effect disappears when the particles become too widely spaced, causing the pressure of the water droplet to spontaneously wet the substrate.