Microscale tracking of unconstrained moving multiphase contact lines via a capacitance sensor array

Abstract

The ability to track the spatial location of a moving multiphase contact line in time significantly enhances understanding into the fundamental physics behind biological processes, condensation- and evaporation-based heat transfer, and a host of other important phenomena. In this work, we present a capacitance-based microdevice capable of tracking a moving phase interface at the microscale for an unconstrained liquid droplet. The microdevice is comprised of an array of planar interdigitated electrodes beneath a thin insulating polymer layer. Monitoring changes in capacitance with time facilitates sensing of the multiphase contact line location and speed as it passes over each capacitance sensor in turn. This capacitive sensing scheme is designed to be highly sensitive and noninvasive to the droplet under study. Results show the speed and location of moving multiphase contact lines for advancing and receding water droplets are indeed able to be detected through the change in capacitance signals in the sensor array. It is also shown that the array can be used to measure the advancing and receding contact angles of unconstrained droplets.