Phase Selection in Capillary Breakup in AC Electric Fields

We study the detachment of conductive aqueous drops in ambient oil from an electrode in the presence of ac electric fields. Making use of the electrowetting effect, we determine the charge of the detached sessile drops. Drops are found to be discharged at high ac frequency in line with earlier predictions. At low frequencies, we find a wide but unexpected charge distribution displaying a frequency-dependent nonzero minimum charge. This observation is explained in terms of the stabilization of capillary bridges in electric fields, which prevents the hydrodynamic pinch-off for certain phases of the ac field.