Speed and location tracking of moving multiphase interfaces via a capacitance microsensor array during droplet evaporation

Abstract

Knowledge of the location and speed of a moving multiphase contact line provides significant and valuable insight into the fundamental physics behind condensation- and evaporation- based processes such as occur in high heat flux thermal management solutions. From an application perspective, this information can be leveraged to ascertain and enhance device design and performance of phase change-based cooling processes. In this work, we present a capacitance-based phase interface sensing approach capable of measuring the location and speed of a moving multi-phase interface at the microscale, evaluate the impact of substrate material on its performance, and demonstrate its ability to function at elevated temperatures during water droplet evaporation. The sensing is accomplished via an array of planar interdigitated electrodes upon either a doped semiconductor or dielectric substrate. Measuring capacitance changes with time facilitates sensing of the contact line as it passes over each electrode pair. This capacitive sensing scheme is noninvasive to the system under study, allowing its implementation into many types of existing hardware and devices and does not require optical access to the phase change area of the device. Results for unconstrained water droplets are presented, and it is shown that the choice of substrate material has a marked impact on sensing behavior in terms of sensor coupling. Finally, data for the moving multiphase contact line of an evaporating water droplet is presented to demonstrate functionality at elevated temperatures and during a dynamic heat transfer process.

Keywords

• Moving contact lines
• Phase change
• Evaporation
• Droplet detection
• Capacitance sensing