Superhydrophobic surfaces by hybrid raspberry-like particles.

Surface roughness on different length scales is favourable for superhydrophobic behaviour of surfaces. Here we report (i) an improved synthesis for hybrid raspberry-like particles and (ii) a novel method to obtain superhydrophobic films of good mechanical stability. Polystyrene spheres with a diameter of 400 nm-1 microm are decorated with silica colloids < 100 nm in size, thus introducing surface asperities on a second length scale. To improve mechanical resistance, we then coated the polystyrene core and attached silica colloids with a smooth silica shell of 10 nm to 40 nm thickness. All three steps of this synthesis procedure can be sensitively tuned so that the average size and number of the silica colloids as well as the morphology of the resulting raspberry particles can be predicted. As the particles disperse in water, either monolayers can be prepared by dip coating or multilayers by drop casting. Although mechanically stable, the shells are porous enough to allow for leakage of molten or dissolved polystyrene from the core. In tetrahydrofuran vapour polystyrene bridges form between the particles that render the multilayer-film stable. Leaked polystyrene that masks some asperities can be removed by plasma cleaning. Surface roughness on larger scales can be tuned by the drying procedure. The films are hydrophobized by silanization with a semi-fluorinate silane.