Calculation of DEP and EWOD Forces for Application in Digital Microfluidics

Abstract

Two primary methods for electrostatically actuating microdroplets in channels currently exist: dielectrophoresis (DEP) for electrically insulating fluids and electrowetting on dielectric (EWOD) for conducting fluids. In each case, a transverse electric field is used to create an electrostatic pressure, giving rise to the transport of individual liquid slugs. This paper examines the nature of the force distribution for both EWOD and DEP actuated droplets. The effects of system parameters such as contact angle and electrode length on the shape of the force density and its net integral are considered. A comparison of the scaling properties of DEP and EWOD for applications in digital microfluidics is presented. The net DEP force is shown to be strongly peaked when a droplet interface is located near the edge of a charged electrode and reduces to the well-known lumped parameter model in the appropriate limits. The effect of electrode spacing is seen to have an inversely proportional effect on the force experienced by the droplet, and the effect of increasing droplet contact angle is observed to increase the net force on the droplet.