Wetting characteristics of ZnO smooth film and nanowire structure with and without OTS coating

ZnO is an important material that is used in a variety of technologies including optical devices, sensors, and other microsystems. In many of these technologies, wettability is of great concern because of its implications in numerous surface related interactions. In this work, the effects of surface morphology and surface energy on the wetting characteristics of ZnO were investigated. ZnO specimens were prepared in both smooth film and nanowire structure in order to investigate the effects of surface morphology. Also, a hydrophobic octadecyltrichlorosilane (OTS) coating was used to chemically modify the surface energy of the ZnO surface. Wettability of the surfaces was assessed by measuring the water contact angle. The results showed that the water contact angle varied significantly with surface morphology as well as surface energy. OTS coated ZnO nanowire specimen had the highest contact angle of 150°, which corresponded to a superhydrophobic surface. This was a drastic difference from the contact angle of 87° obtained for the smooth ZnO film specimen. In addition to the initial contact angle, the evolution of the water droplet with respect to time was investigated. The wetting state of water droplet was analyzed with both Wenzel and Cassie-Baxter models. Spontaneous and gradual spreading, together with evaporation phenomenon contributed to the changing shape, and hence the varying contact angle, of the water droplet over time.