Energy conversion during electrically actuated jumping of droplets

Abstract

Many industrial technologies, such as condensation cooling and fuel cells, require solid-liquid separation. Electrowetting is a very effective method of inducing droplets to detach from hydrophobic surfaces, and it is very convenient to control. The jumping of droplets excited by an electric field depends on the conversion of surface energy into kinetic energy and other forms of energy. At present, there is still a lack of in-depth research on this process. In this study, a high-speed camera is used to capture the jumping motion of a droplet on a hydrophobic surface under the actuation of electrowetting, and the threshold voltage that causes the droplet to detach is estimated based on the changes in contact angle and droplet shape. A self-written Matlab program is used to analyze and calculate the various forms of energy in the process of droplets detaching and subsequent bouncing. The results show that there is an obvious coupling relationship between the kinetic energy and potential energy of the droplet’s center of mass during the flight of the droplet from the surface. The vibrational kinetic energy and surface potential energy also show a certain coupling relationship during the flight phase. The internal dissipation caused by the viscosity of the droplet increases with the droplet oscillation amplitude increasing, and decays with time. Because it can cause the droplet to oscillate and deform and create more surface energy, AC pulses are more efficient than direct current in the droplet bounce. By revealing the energy conversion and dissipation mechanism in the process of droplet jumping driven by electrowetting, a theoretical reference is provided for the application of this technology in solid-liquid separation and three-dimensional digital microfluidics.