The Use of Impedance Spectroscopy in Characterizing Low Voltage Electrowetting-on-dielectric

Being able to control fluid shape and flow using an external electric field alone without any mechanical parts, electrowetting-on-dielectric (EWOD) has drawn a lot of attention from researchers which leads to the the invention of many interesting applications. In the laboratory, EWOD characterization is generally conducted using direct optical observation of the droplet. While being a straight-forward method, optical techniques are laborious, slow, subject to optical distortions, and impractical in devices where the electrowetted droplet is a functional part of a larger system. In such cases, having an integrated means to interrogate the droplet configuration quickly and accurately would be highly desirable. As most EWOD experimental setups/devices have a metal-insulator-metal structure, it is possible to characterize the wetting process using electrical measurements. Prior efforts in characterizing EWOD electrically exhibit limitations in: 1) the dielectric layer used were relatively thick; 2) overly simplistic assumptions were made regarding the droplet shape. Because electrical characterization of EWOD is beneficial in many ways, we have investigated methods to expand its use and increase its flexibility so it could be applied to various kinds of experiments and applications. In this work, we first experimentally investigated the impedance of various EWOD configurations as a function of frequency, helping researchers to develop a framework for characterizing the impedance of an EWOD experiment using electrode properties and material parameters. Secondly, we developed methods to use electrical measurements in conjunction with known capillary bridge geometries to accurately determine unknown parameters in electrowetting (e.g., contact angle, liquid volume, interfacial tension, etc.). Lastly, we designed, built, and tested a capacitance measurement system capable of high-speed time-resolved measurements of electrowetting behavior.