Condensation Droplet Distribution Regulated by Electrowetting

This paper presents a visualization of condensation droplet distribution affected by the electrowetting-on-dielectric (EWOD) approach. A single-side double-layer-electrode design (grid wire, thin wire, and thick wire) and coplanar-electrode design (zigzag) are discussed. Side-by-side experiments with applied 40 V DC electric potential are carried out to compare droplet distribution between identically designed charged and uncharged devices. The uncharged devices show a random droplet distribution, whereas charged devices have a regulated distribution based on the designed patterns. As droplets on the electrode boundaries become larger, they are likely to slide away and stay in electrode-free regions. The droplets “sit” inside the grid wires and distribute vertically along thin and thick wires. On the coplanar-electrode zigzag device, droplets are distributed vertically. The charged surfaces lead to a faster droplet growth rate and more dispersed droplet distribution. This phenomenon accelerates the shedding frequency of the droplets and frees up more areas for small droplets to nucleate and grow. The first shedding moment of the charged surfaces occurs earlier than the uncharged ones for all types of EWOD devices. The detected droplet shedding diameter ranges from 1.2 mm to 2.5 mm in this study. The number of large droplets is found greater on the charged devices compared with the uncharged devices and theoretical model. The work presented in this paper introduces a novel approach to actively influence droplet distribution on microfabricated condensing surfaces and indicates great potential for improving the condensation heat transfer rate via EWOD.