Chemically guided topography in alkylsilane- and oligosiloxane-modified silica nanoparticle coatings: from very hydrophobic surfaces to "pearl" bouncing droplets

The surface of silica nanoparticles has been modified using four sets of reagents: trimethoxysilanes (TMS), methoxysilanes (MS), dimethoxysilanes (DMS), and polydimethylsiloxanes (PDMS). Characterization of the organic loading and structure has been carried out. Glass slides have been coated with the organosilicas, and the wetting/topographic properties studied. Especially for the TMS organosilica series, the wetting behaviour of the coatings changes from very hydrophobic to superhydrophobic and finally to "pearl" bouncing droplets with contact angle of 180°, as the length of the silane alkyl side chain increases. The length of the side chain modifies not only the surface chemistry but also the aggregation state of the nanosilica, and thus controls the coating topography at the nano-, submicro and micrometre scales. This has been called "chemically guided topography", and it produces a hierarchical structure dependent on the silica surface coverage, leading to different wetting responses. The derivatization of A200 with short to medium chain length alkyltrimethoxysilanes produces organosilica coatings with a characteristic hierarchical surface roughness that presents tunable wetting responses from very hydrophobic to "pearl" bouncing droplets.