On the Connection between Dielectric Breakdown Strength, Trapping of Charge, and Contact Angle Saturation in Electrowetting

Abstract

Electrowetting on dielectric (EWOD) is simulated by solving the equations of capillary electrohydrostatics, by the Galerkin/finite element method. Aiming to provide reliable predictions of the voltage dependence of the apparent contact angle, close to or beyond the saturation limit, special attention is given in the treatment of the dielectric properties of the solid dielectric where the liquid sits. It is proposed that in regions where the electric field strength locally exceeds the material breakdown strength, the dielectric locally switches to a conductor. Without using any fitting parameter, the implementation of the proposed phenomenological idea realized a surprising matching of published experimental data concerning materials ranging from SiO₂ to Parylene N and Teflon. Charge trapping is naturally connected to the field-induced transition, and its distribution as well as its dependence on the applied voltage is calculated.