5 - EWOD Microsystems

Abstract

This chapter provides an overview of electrowetting on dielectric (EWOD) microsystems. There are two types EWOD microsystems—open and covered. Open systems where the sessile droplet is sitting freely on a horizontal solid substrate, and covered systems where the droplet is confined between two plates. Each of these systems has their own advantages. Drop dispense, motion, and splitting are easier in covered EWOD systems, whereas mixing and evaporation are preferably performed in the open configuration. With some hybrid designs, these two types of EWOD are interfaced such that droplet motion between a covered and an open region is possible under electrowetting actuation. If a conductive droplet is located on the boundary between an actuated and a nonactuated electrode, an electrowetting force is applied on the contact line located above the actuated electrode, and a capillary force is exerted on the contact line located on the lyophobic surface. The resultant of the forces parallel to the surface is directed towards the electrically actuated region. The droplet is out of equilibrium, and if the resulting force is sufficient to overcome hysteresis, the droplet moves. In digital microfluidic systems, a droplet moves from one location on the chip to the next incrementally. Because inertia forces are small, the droplet accelerates very quickly, reaches a nearly steady velocity, and decelerates abruptly. Velocity depends on the strength of the electrocapillary forces. It is then expected that velocity of the droplet is directly related to this force.