Frequency-dependent resonance and asymmetric droplet oscillation under ac electrowetting on coplanar electrodes

Sessile droplet oscillations in electrowetting on dielectric with a coplanar-electrode configuration are studied experimentally under the actuation of ac voltage with different frequencies. It was found that the experimental resonance frequencies and the number of lobes at different resonance modes agree reasonably well with a previous linear analysis. Oscillations of contact width and droplet height are in-phase at resonance modes P_2n+2 while out-of-phase at P_2n with n = 2, 4, 6,…. At certain critical frequencies, the droplet oscillations are very weak and switch from in-phase (out-of-phase) to out-of-phase (in-phase). For the oscillations after resonance frequency but before critical frequency, at low frequency, the large amplitude oscillation of the contact line deforms it from a circle to having lobes; the number of lobes increases with the frequency and their position alternates in the azimuthal direction, through periodical droplet spreading and receding. For the oscillations after critical frequency but before resonance frequency, the droplet oscillation demonstrates droplet waggling with an obvious contact line at normal and abnormal stops, due to the contact line pinning at low frequency, or the transportation of lobes on the droplet surface from one end to the other at high frequency. These asymmetric oscillations will produce a more chaotic fluid flow inside the droplets than symmetric oscillations and could be used purposely to enhance mixing in droplet-based micro-fluidics. (paper)