Liquid Motion Induced by Electrocapillary Action at Solid Metal-Liquid Interfaces

It is shown that cyclic mechanical displacements are induced in a variety of aqueous solutions and organic liquids when alternating voltages are applied across the interfaces between them and solid metal electrodes. The displacements, in the picometer-nanometer range, are proportional to the applied voltage and are determined using an interferometer technique which detects the movement of a mirror floating on the free surface of the liquid above the interfaces. The displacements, particularly in the case of a stainless steel-glycerol interface, increase considerably when steady bias voltages are applied. The experiments are in agreement with the predictions of the Lippmann electrocapillary equation for interfaces and complement those where stresses or strains generated in the solid electrode rather than the liquid side of the interfaces have been measured. Since similar displacements have been measured at solid as well as liquid contacts to metal electrodes, it is concluded that the Lippmann equation predicts quasi-piezoelectric phenomena at all condensed matter interfaces.