UNSTEADY ELECTROPHORETIC MOTION OF A NONSPHERICAL COLLOIDAL PARTICLE IN AN OSCILLATING ELECTRIC-FIELD

The oscillatory motion of an electrically charged non-spherical colloi dal particle in an oscillating electric field is investigated. The par ticle is immersed in an incompressible viscous fluid and assumed to ha ve a thin electric double layer. For moderate-aspectration spheroids a nd cylinders, a simple algebraic expression is derived that accurately describes oscillatory electrophoretic particle motion in terms of the steady Stokes resistance, added mass, and Basset force. The effects o f double-layer conduction and displacement currents within dielectric particles are included. The results indicate that electroacoustic meas urements may be able to determine the zeta-potential, dielectric const ant, surface conductivity (and microstructural information contained t herein), size, density, volume fraction, and possibly shape of non-sph erical particles in a dilute suspension. A simple formula is obtained for the high-frequency electrical conductivity of a dilute suspension of colloidal spheroids with arbitrary charge and dielectric constant; only the added mass and Basset force are required and the requisite pa rameters are given. The result is needed for electroacoustic measureme nts but it may also be independently useful for determining the dielec tric constant, surface conductivity, volume fraction, and possibly the shape of non-spherical particles in a dilute suspension. Electroacous tic energy dissipation is described for a dilute colloidal suspension. It is shown that resistive electrical heating and viscous dissipation occur independently. Electrical and viscous dissipation coefficients that characterize the order volume fraction contributions of the suspe nded particles are calculated; the electrical dissipation coefficient is O(1) for all oscillation frequencies, whereas the latter vanishes a t low- and high-frequencies. The fluid motion is shown to be a superpo sition of unsteady, viscous and potential flows past an oscillating pa rticle with no applied electric held. The electro-osmotic flow field i s insensitive to particle geometry and qualitatively different from th e flow past an oscillating particle with no applied field.