Not only dielectrophoresis: The crucial role of electrokinetic phenomena in a dielectric particle’s response to an oscillating electric field

Abstract

Dielectrophoresis (DEP) is frequently analyzed in terms of a particle’s effective dipole moment (EDM). However, in a non-uniform oscillating electric field there are other electrokinetic (EK) phenomena, such as induced-charge electrophoresis (ICEP) and higher order corrections to electrophoresis (EP) that contribute to the particle’s time-averaged motion. We analyze the situation for a highly charged dielectric particle with a thick electric double layer for a general non-uniform external field. By applying perturbation theory consistently to the second-order in terms of the external electric field’s strength, we show that the Maxwell stress tensor (MST) evaluated on the particle’s surface naturally includes both DEP and EK effects, whereas the EDM approach only yields the DEP contribution. Notably, when the MST is calculated in the neutral and uniform far field, as opposed to the particle’s surface, this yields only the DEP contribution as given by the EDM. Importantly, electrokinetic phenomena are significant for highly charged dielectric particles and may even dominate the dielectrophoretic response at low frequencies. Under these conditions, the Clausius–Mossotti factor can be a misleading parameter to predict whether a particle is attracted to, or repelled from, regions with a high electric field strength.