Microfluidic mixing through electrowetting-induced droplet oscillations

Abstract

We used electrowetting to trigger self-excited oscillations of millimeter-sized sessile droplets of water-glycerol mixtures in a viscosity range from 1to65mPas. During the oscillations the contact angle of the droplets varied periodically between ≈130° and 80° with a frequency between 10 and 125s−1, depending on the viscosity and the drop size. By initially staining drops partially with fluorescent dye, we found that the liquid within the drop is completely mixed within 100–2000 oscillation cycles for low and high viscosities, respectively. Compared to pure diffusion, droplet oscillations accelerated mixing by approximately two orders of magnitude for millimeter-sized droplets.