Electrowetting Actuation for a Sessile Liquid Drop: Experiments and Simulations

In this paper, experiments and numerical simulations are performed to study the effects of electric field on the contact angle of a sessile liquid drop resting on a substrate in electrowetting-on-dielectric (EWOD) application. In the experiments for studying the electrowetting, a mercury droplet of 20μL was dispensed manually on a soldermask-coated PCB (Printed Circuit Board) and different values of AC voltage were applied between the droplet and the insulator. High quality images were captured using a CCD camera in all experiments and a program was developed using the MATLAB software for image-processing purposes to obtain the contact angle and other geometrical parameters of the droplet. A numerical model was also used to simulate the drop deformation under an electric field. The continuity and momentum equations along with an equation for tracking the liquid free surface were solved. The free surface advection and reconstruction were performed based on the volume-of-fluid method using Youngs’ algorithm. To evaluate the effect of the electric field on the free surface, the electrostatic potential was first solved for the entire computational domain. Next, the electric field intensity and the surface density of the electric charge were calculated on the free surface after which the electric force could be determined. Calculated droplet shapes agreed well with those of the experiments.