Electrorotation instability of a liquid bridge

We experimentally study the behavior of an oil droplet sandwiched between two ITO coated glass electrodes in a DC electric field. Above a threshold electric field, the contact line contour changes from circular to a steady flower-like one. As the field strength increases, the wave becomes unsteady and begins to rotate. It is observed that the stable mode of the wavy contact line is different if the field is applied as a pulse or gradually increased in small steps, which suggests that the charge relaxation time is important. The amplitude of the wave is related to the conductivity of the drop fluid, which is controlled by adding organic electrolytes. The physical mechanism of the instability is yet to be explained.