Facile fabrication and mechanistic understanding of a transparent reversible superhydrophobic - superhydrophilic surface

We report a simple, inexpensive and rapid method for fabrication of a stable and transparent superhydrophobic (TSHB) surface and its reversible transition to a transparent superhydrophilic (TSHL) surface. We provide a mechanistic understanding of the superhydrophobicity and superhydrophilicity and the reversible transition. The proposed TSHB surface was created by candle sooting a partially cured n-hexane + PDMS surface followed by washing with DI water. The nano/microscopic grooved structures created on the surface conforms Cassie-Baxter state and thus gives rise to superhydrophobicity (water contact angle (WCA) = 161 degrees +/- 1 degrees). The TSHB surface when subjected to oxygen plasma develops-OH bonds on the surface thus gets transformed into a TSHL surface (WCA < 1 degrees). Both surface chemistry and surface morphology play important roles for the superhydrophobic to superhydrophilic transition. In the Cassie-Baxter relation for a composite surface, due to the capillary spreading of liquid in the nano/micro grooves, both theta(1), theta(2) = 0, thus giving rise to complete wetting. Rapid recovery of superhydrophobicity from superhydrophilicity was achieved by heating the TSHL surface at 150 degrees C for 30 min, due to a much faster adsorption of the-OH bonds into the PDMS. Thus it is possible to achieve reversible transition from TSHB to TSHL and vice versa by exposing to oxygen plasma and heat, respectively.