Electrowetting on Superhydrophobic Surfaces: Present Status and Prospects

Electrowetting devices with an initial superhydrophobic water contact angle (>150°) have now been demonstrated on a variety of structured substrates. These substrates are more complex than a conventional superhydrophobic surface since electrowetting requires an electrical conductor that is coated with a high-performance dielectric and a hydrophobic fluoropolymer. Substrate structures that have been studied include silicon nanoposts and nanowires, carbon nanofibers and nanotubes, and polymer microposts. Even though these structured surfaces are geometrically diverse, there are several consistencies in electrowetting behavior for all these platforms. As an electrowetting bias of 10's of volts is applied between a saline drop and the substrate, the macroscopically observed contact angle is typically decreased from >150° to ∼100°. As the voltage is increased an electromechanical force promotes capillary wetting between the substrate structures, and the saline drop transitions from the Cassie state to the Wenzel state. The Wenzel state presents a new energy minimum for the system, and in all current experiments the wetting is irreversible. Transition from the Wenzel state back to the Cassie state has been demonstrated by means of liquid boiling or addition of a second non-polar liquid. The importance of these recent investigations includes the dynamic tuning of the wetting on a superhydrophobic surface, and improved understanding of electrowetting on, and into, structured surfaces.