Effects of interparticle electric interactions on dielectrophoresis in colloidal suspensions

Using our recently published statistical theory concerning the phase diagram of a low-conducting colloidal suspension subject to uniform electric fields [Phys. Rev. E 52, 1669 (1995)], we examine how the long-range electric-field-induced interparticle interactions affect the spatial arrangement of particles in such a suspension under the action of a nonuniform ac electric field. We find the conditions under which the resulting dielectrophoresis in nonuniform electric fields is accompanied by an electric-field-induced phase transition in the suspension. Moreover we predict that, in the case of positive dielectrophoresis, the particles will form chainlike aggregates aligned parallel to the electric field lines and attracted towards the higher electric-field region; whereas, for negative dielectrophoresis, the particles will form disklike aggregates aligned perpendicularly to the electric-field lines and repelled from the higher electric-field region. The theory also provides some insight regarding the dependence of the particle aggregation on the frequency of the applied nonuniform ac electric field. The predictions of the theory are consistent with the characteristic patterns of cell aggregation observed previously in high-gradient electric fields generated in microelectrode systems.