Formation of liquid columns on liquid-liquid interfaces under applied electric fields

Formation of multiple liquid columns on an initially horizontal, stationary liquid-liquid interface has been studied with an apparatus that allowed for a large distance between the interface and the parallel electrodes used and a vertically applied, direct-current electric field. Interface instability led to the formation of columns in systems consisting of water, toluene, and isopropanol in various concentrations. The liquid columns were drawn from the aqueous phase by the electric force with their upper ends either free or anchored on the upper electrode, depending on the operating conditions and physical properties of the system. Under the action of the electric field, the columns moved and twisted irregularly on the interface, with rotation about the column axis, until they disintegrated into streams of droplets. The formation and morphology of the liquid columns were determined according to the strength of the electric field and the physical properties of the fluids. The total number of columns and their average height, diameter, and slenderness ratio were found to increase with an increase in applied voltage. Lowering the interfacial tension, reducing the density difference between the two phases, and increasing the permittivity of the conducting phase decreased the critical voltage for interface instability onset and the threshold voltage for column formation. A linear stability analysis was carried out based on a model developed by Melcher and Smith (25), extended for mechanical motions on the order of the viscous diffusive length of a small perturbation. The model includes viscosity, relative electric permittivity, electrical conductivity, and interfacial free charge. The results overestimated the critical voltage for instability onset but they shed light on the role played by interfacial free charge on the stability of the system. In particular, a nonzero interfacial free charge can have a stabilizing effect on a liquid-liquid interface formed by fluids whose electrical properties are not drastically different.