Single step fabrication of nano-structured superhydrophobic surfaces showing angle dependent colours

Abstract

Superhydrophobic surfaces show extraordinary water-repellent properties with low drag for fluid flow due to reduced liquid-solid contact area. Due to high contact angle and low contact angle hysteresis these surfaces also show self-cleaning effect. In nature different plants and leaves, such as Lotus leaf and Rose petals show superhydrophobic behaviour due to wax coated micro/nano hierarchical structures on their surfaces. In this work, we report fabrication of superhydrophobic surfaces on silicon substrate using a one step process. This is a wafer scale large area fabrication technique for superhydrophobic surfaces. DRIE (Deep Reactive Ion Etching) technique was used to fabricate the nano structured extremely water repellent surfaces. Two different fabrication approaches have been followed in this work. In one approach the DRIE was carried out directly on silicon surface and in the second approach DRIE was done after spin coating alumina nanoparticles dissolved in ethanol on silicon surface. In both the processes, DRIE was done for a different number of etch cycles. The width of the nano-structures formed after DRIE process varies in the range of 300 nm to 500 nm. Contact angle has been measured and compared for the superhydrophobic surfaces fabricated using the above two approaches. Contact angle as high as 170°±1° was measured and less than 2° contact angle hysteresis was observed for water droplet. Also an angle dependent colour change phenomena, which was observed for the nano structured silicon substrates obtained using the second approach at different viewing angles is reported.