Electrowetting: Electrocapillarity, saturation, and dynamics

Electrowetting is an electrocapillary phenomenon, i.e. the surface charge generated at the solid-liquid interface through an external voltage improves the wettability in the system. The Young-Lippmann equation provides the simplest thermodynamic framework and describes electrowetting adequately. Saturation, i.e. the reduced or nullified effectiveness of the external voltage below a threshold contact angle value, was and remains the most controversial issue in the physics of electrowetting. A simple estimation of the limits of validity of the Young model is obtained by setting the solid-liquid interfacial tension to zero. This approach predicts acceptably the change in electrowetting mechanism but not the minimal value of the contact angle achievable during electrowetting. The mechanism of saturation is, in all probability, related to charge injection into the dielectric layer insulating the working electrode but physical details are scarce. Surface force and spectroscopic techniques should be deployed in order to improve our understanding of the surface charging of insulators immersed in conductive liquids. Electrowetting in solid-liquid-liquid systems is generally more effective and robust. Electrowetting offers new ways of studying the dynamics of liquid movement as it allows selective changes in the wettability of the system.