Microstructure-alone induced transition from hydrophilic to hydrophobic wetting state on silicon

Surface hydrophobicity is primarily attained through the use of low surface energy materials. Experimental attempts to turn hydrophilic surfaces to hydrophobic have consisted of coating and thin film deposition. However, in many applications low surface energy materials and coatings are not practical, though hydrophobicity is still desired. In this paper, we demonstrate the transition from hydrophilic to hydrophobic wetting states on an intrinsically hydrophilic surface (contact angle less than 45°) using only surface microstructuring. The surface microstructures consist of re-entrant microcavities which interfere with the complete wetting of the surface, causing a liquid droplet to sit on the surface in a Cassie wetting state. The microstructures were fabricated on a silicon-on-insulator (SOI) wafer through steps of photolithography, etching, and bonding. Contact angle measurements demonstrated the ability of the microfabricated surfaces to sustain large contact angles above 100°, compared to a bare silicon surface which has a contact angle of 40°. Energy-dispersive X-ray spectroscopy showed silicon to be the only chemical element on the surface, while optical observations with an inverted microscope hinted to the existence of a Cassie wetting state.