Manipulation of bio-particles by means of nonuniform AC electric fields

In this paper we examine the motion and behavior of particles suspended in aqueous solutions subjected to non-uniform ac electric fields. The particles can move due to forces exerted over them, or due to the motion of the surrounding liquid. In the first case, we have dielectrophoresis, due the action of ac electric fields over polarizable particles. The high strength electric fields often used in separation systems can give rise to fluid motion, which in turn results in a viscous drag on the particle. The electric field generates heat, leading to volume forces in the liquid. Gradients in conductivity and permittivity give rise to electrothermal forces; gradients in mass density to buoyancy. In addition, non-uniform ac electric fields produce forces on the induced charges in the diffuse double layer on the electrodes. This gives a steady fluid motion known as ac electroosmosis. We also discuss the effects of Brownian motion in this context. We calculate the different forces and displacements and compare them for a simple system consisting of a saline solution subjected to a traveling wave electric field. This example provides scaling laws of a wider applicability.