Fluid Flow and Mixing within Drops in Ac Electrowetting

Electrowetting generates flow patterns inside drops when AC voltages are employed. For AC frequencies around the drop eigenfrequency (typically O(1kHz) or less), capillary waves emanate from the contact line giving rise to a net motion in the bulk of the drop. This flow can be described by a capillary wave-driven Stokes drift. At higher AC frequencies (above 10kHz, depending on liquid conductivity) electrical currents generate nonuniform Joule heating, giving rise to temperature gradients and, consequently, gradients in conductivity and permittivity. We show how the electric field acting on these gradients can generate fluid flow (Electrothermally induced flows).