A Model of Electrowetting, Reversed Electrowetting and Contact Angle Saturation

TL;DR

This paper introduces a comprehensive electrowetting model that explains contact angle saturation, predicts reversed electrowetting, and aligns well with experimental data across various setups and frequencies.

Contribution

It presents a generalized electrowetting model incorporating energy minima and voltage dependencies, offering new insights into contact angle saturation and reversed electrowetting phenomena.

Findings

Model predicts contact angle saturation at high voltages.

Reversed electrowetting regime is theoretically possible.

Model aligns with experimental AC and DC electrowetting data.

Abstract

While electrowetting has many applications, it is limited at large voltages by contact angle saturation - a phenomenon that is still not well understood. We propose a generalized approach for electrowetting that, among other results, can shed new light on contact angle saturation. The model assumes the existence of a minimum (with respect to the contact angle) in the electric energy and accounts for a quadratic voltage dependence ~U^2 in the low-voltage limit, compatible with the Young-Lippmann formula, and a ~1/U^2 saturation at the high-voltage limit. Another prediction is the surprising possibility of a reversed electrowetting regime, in which the contact angle increases with applied voltage. By explicitly taking into account the effect of the counter-electrode, our model is shown to be applicable to several AC and DC experimental electrowetting-on-dielectric (EWOD) setups. Several features seen in experiments compare favorably with our results. Furthermore, the AC frequency dependence of EWOD agrees quantitatively with our predictions. Our numerical results are complemented with simple analytical expressions for the saturation angle in two practical limits.