Underwater Wenzel and Cassie oleophobic behaviour

Abstract

Underwater wettability and wetting transitions of non-polar liquids with rough solid surfaces are herein presented. Here, we demonstrate that a hydrophobic/oleophilic surface when immersed in water can result in an oleophilic or “Cassie” state of oil-wetting. This was achieved by utilizing respectively a diamond like carbon (DLC) coating on a cotton substrate and a combination of DLC and zinc oxide (ZnO) nanorods embedded into the structure. Moreover by increasing the time of immersion the oleophilic state reverted to a Wenzel state evolving to Cassie's regime. The mechanism of the transition was identified with the diffusion of the air into the water from the trapped air pockets of the substrate interface. Finally, switchable wettability of ZnO/DLC coated substrates allows a promising oil–water separation use.