Mass Transport Phenomena in Superhydrophobic Surfaces

We present results of a droplet placed on a controlled super-hydrophobic surface cooled underneath by a thermal electrical cooler to demonstrate quick change in contact angles from the Cassie composite contact state to the Wenzel wetting contact state. The measured contact angles are compared with the theoretical predictions of Cassie’s and Wenzel’s equations and found to be consistent. The actual details of the transition phenomena are observed under a microscope through a specially designed one-dimensional micro-channel with concaved structures at the two sidewalls. It is found that the temperature gradient enhanced mass transfer can cause a rapid condensation in the air-filled cavities, which is believed to be the possible mechanism to trigger the energy state transition and explain instabilities of super-hydrophobic surfaces at the Cassie state. The phenomenon of mass transport into micro and nanocavities is important in understanding the nature of nano-structured super-hydrophobic surfaces.