Preventing the Cassie-Wenzel transition using surfaces with noncommunicating roughness elements.

Control and switching of liquid droplet states on artificially structured surfaces have significant applications in the field of microfluidics. The present work introduces the concept of using structured surfaces consisting of noncommunicating roughness elements to prevent the transition of a droplet from the Cassie to the Wenzel state. The use of noncommunicating roughness elements leads to a confinement of the medium under the droplet in its Cassie state. Transition to the Wenzel state on such surfaces requires expulsion of this confined medium, which offers significantly increased resistance to the Wenzel transition unlike surfaces consisting of communicating roughness elements. This enhances the robustness of the Cassie state and significantly minimizes the possibility of the Cassie-Wenzel transition. In the present work, the resistance of a surface to the Wenzel transition is measured in terms of the electrowetting (EW) voltage required to trigger this transition. It is seen that surfaces with noncommunicating roughness elements (cratered surfaces) require significantly higher voltages to trigger the Wenzel transition than corresponding surfaces with communicating roughness elements. The findings from the present work also indicate that EW-induced droplet morphology control characteristics show a strong dependence on the nature of the roughness elements (communicating versus noncommunicating). Different aspects of droplet morphology and EW-induced state transition control on surfaces with noncommunicating roughness elements are analyzed; it is seen that such surfaces offer significant possibilities for the development of robust superhydrophobic surfaces.