Microfluidic mixing through electrowetting-induced droplet oscillations

We used electrowetting to trigger self-excited oscillations of millimeter-sized sessile droplets of water-glycerol mixtures in a viscosity range from 1 to 65 mPa s. During the oscillations the contact angle of the droplets varied periodically between approximate to 130 degrees and 80 degrees with a frequency between 10 and 125 s(-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.