Date of this Version
Ems,H, Microstructure Alone Induced Wetting Transition From Hydrophilic to Hydrophobic on Silicon and Graphene, (2015).
In the present work, transition from hydrophilic to hydrophobic wetting states for an intrinsically hydrophilic surface (contact angle less than 45 degrees) using only surface microstructuring is presented. The surface microstructures are re-entrant microcavities (inverted trapezoidal microstructures) which promote air entrapment below the water droplet causing a Cassie wetting state as opposed to a Wenzel state where the surface is completely wetted. The microstructures were fabricated on a Silicon-On-Insulator (SOI) wafer through steps of deposition, photolithography, etching, and bonding. Contact angle measurements demonstrated the ability of the microfabricated surfaces to sustain large contact angles above 100°, in comparison to a bare Silicon surface which has an intrinsic contact angle around 40°. Energy-dispersive x-ray spectroscopy showed Silicon to be the only chemical element on the surface, meaning free from contaminants that were possible from etching and handling. Optical observations with an inverted microscope hinted to the existence of a Cassie wetting state. In the second part of the thesis, graphene is formed on the inverted trapezoidal microstructures through steps of oxidation, deposition, and thermal evaporation. After fabrication techniques are performed, a thin layer of graphene is left on top of the oxide layer. Using re-entrant microcavities the contact angle transitioned from 77.5 degrees to 91 degrees. Raman Spectra and EDS proved the presence of monolayer graphene and the presence of Nickel, Silicon, and Oxide respectively.
Advisor: Sidy Ndao