Controlling flow patterns in oscillating sessile drops by breaking azimuthal symmetry

We study time-averaged flows within sessile drops that oscillate under the influence of an AC voltage applied in electrowetting configuration. We show that the average flow velocity in the azimuthal plane correlates with the eigenmodes of the drop in the polar direction and—most importantly—we demonstrate that the azimuthal symmetry of the flow fields can be broken by introducing pinning sites along the contact line of the drop. We anticipate that the controlled introduction of azimuthal vortices increases the mixing efficiency inside the droplet.