How electrostatic fields change contact angle in electrowetting

Abstract

The electrowetting phenomenon is investigated in the viewpoint of the classical electrostatics. Special attention is devoted to excavating the influence of excess charge induced at the edge region of a droplet. For this, the electrostatic field around an infinite wedge is analyzed, assuming the droplet as a perfect conductor. It is shown that the Maxwell stress is concentrated on the small region comparable to the thickness of the thin dielectric film beneath the liquid droplet. On the basis of the macroscopic balance condition of the horizontal-force components at the three-phase contact line, the conventional electrowetting equation is derived. This result suggests that macroscopic changes of contact angle originated from the electrostatic force, rather than from the change of the interfacial tension at the droplet-dielectric interface. Moreover, as the apparent contact angle becomes small, the vertical component of the electrostatic force, which would oppose the reduction of contact angle, increases significantly. This can be another possible cause of the limited validity of the conventional electrowetting equation and subsequent occurrence of contact-angle saturation phenomenon.