Abstract: An experimental study on droplet oscillations under AC electrowetting on dielectric (EWOD) with coplanar-electrodes configuration at different input frequencies was conducted. It was found that the droplet resonates at some specific frequencies, which agrees reasonably well with those predicted using a previous linear theoretical analysis, and the resonance modes is named Pn (n=2, 4, …). At resonance modes Pn, the droplet oscillates symmetrically with n/2 peaks appearing on the droplet surface. There exists a critical frequency between the adjacent resonance modes, at which the droplet oscillates symmetrically but very weakly, and the oscillation phase relationship between the contact line width and height of the droplet switches. At frequencies lower than this critical frequency, the contact line will form lobes whose number increases with the order of adjacent resonance mode and whose position alternates periodically in azimuthal direction through droplet spreading and receding. At frequency higher than the critical frequency, droplet oscillates asymmetrically with the transmission of ripples on the droplet surface. It is believed these asymmetric oscillations will produce more chaotic fluid flows inside the droplet than symmetric oscillations and therefore can enhance the mixing efficiency in dropletbased microfluidics.  

Key words: AC electrowetting on dielectric(EWOD), droplet oscillation, microfluidic, mixing