Resonant droplet tensiometry driven by an electric field

Abstract

A submillimeter sized pendant spherical droplet is placed in a nonuniform electric field. Combination of AC and DC electric field generates a frequency modulated dielectrophoretic force. Caused by the dielectric permittivity jump over the liquid-air interface, the force acts directly upon the interface. Thus the droplet is set in to forced axisymmetric oscillations. An optical laser system is used to measure the amplitude and the frequency of the surface oscillations in real time. The droplet radius is measured with a highly accurate imaging system and shape analysis software. The resonant frequency determined from the obtained resonant curves is used to estimate the surface tension of the liquid. Analysis of the resonant curves gives additional information about the viscosity and the supporting surface influence on the droplet's behavior. Our technique operates in two modes: "tensiometric mode" for surface tension measurements and "manipulation mode" for studying and manipulating liquid interfaces via an electric field. In the present work the principles, applications and capabilities of Resonant Droplet Tensiometry driven by an electric field, have being reviewed as well as the latest results and challenges.