The transition between wetting states and novel methods of producing superhydrophobic surfaces

The stability of wetting states (the Cassie state---partial wetting and the Wenzel state---complete wetting) of surfaces with protrusions is determined by comparing the total free energy of the two states. It is found that large area fraction of the topographical features, the intrinsic contact angle and the aspect ratio favor the Cassie state. The transition from one wetting state to the other requires the application of a critical pressure to the meniscus between the surface protrusions, which increases with increasing area fraction and intrinsic contact angle, and decreasing protrusion size. The critical drop size to break away from a surface is also inversely proportional to the feature size.; Three methods are studied to produce texture surfaces with particles that could be converted to superhydrophobic surfaces. One is to produce partially sintered powder compacts. The rough surfaces of these compacts become a superhydrophobic after reaction with OTS (Octadecyltrichlorosilane), but not with dodecanol. The reaction of dodecanol is prohibited inside the compact body, whereas the compact can only be made superhydrophobic after a subsequent heat treatment in air at 170°C. Another method is to produce small Au islands on thermally grown SiO2 by instability. The final islands size is about the 10 times the original film thickness. The procedure can be repeated a number of times to increase the island density and achieve complex surface structures. A third method to produce Al2O3 particles decorated flat glass surface via sedimentation. The pH is carefully controlled between the two IEPs (iso-electric-point), thus the different surface charges produce electrostatic attraction between Al2O3 and glass strong enough to hold the particles from being pulled together by the meniscus during drying. When the surface coverage increases, it is inevitable to have particle clusters on the substrate because of the random particle positions in the slurry, which results very high contact angle. The soluble alumina species in alumina slurries are found to coat glass particles (and substrates) surface; like-wise, soluble silica species coat alumina particles by the measurements of zeta potential.; (111) Si was made superhydrophobic by utilizing the formation of SiO 2 on the Au film deposited on Si at low heating temperature. Heated at 150°C, the roughness of the film progressively increases that leads to the transition from the Wenzel state to the Cassie state after alkylation. The optimal conditions and evolution of the SiO2 protrusion formation are explored. The Si diffuses through the low-angle grain boundaries of Au, and the oxygen counter diffuses in forming SiO2 in the Au film.