Non-uniform ac electric fields induce movement of polarizable particles. This phenomenon, known as dielectrophoresis, is useful to manipulate bioparticles. High electric fields when used in bio-separation systems give rise to fluid motion, which in turn results in a viscous drag on the particle. These fields generate heat, leading to volume forces in the liquid. Gradients in conductivity and permittivity rise to electrothermal forces; gradients in mass density to buoyancy. Also non-uniform ac electric fields produce forces on the induced charges in the diffuse double layer on the electrodes, and the resulting steady fluid motion has been termed ac electroosmosis. The effects of Brownian motion and diffusion are also discussed in this context. The orders of magnitude of the various forces experienced by a submicrometre particle in a model electrode system are calculated. The results are compared with experiments and the relative influence of each type of force is described.

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