EFFECTS OF INTERPARTICLE ELECTRIC INTERACTIONS ON DIELECTROPHORESIS IN COLLOIDAL SUSPENSIONS

Using our recently published statistical theory concerning the phase d iagram of a low-conducting colloidal suspension subject iu uniform ele ctric fields [Phys. Rev. E 52, 1669 (1995)], we examine how the long-r ange electric-field-induced inter-particle interactions affect the spa tial arrangement of particles in such a suspension under the action of a nonuniform ac electric field. We find the conditions under which th e resulting dielectrophoresis in nonuniform electric fields is accompa nied by an electric-field-induced phase transition in the suspension. Moreover we predict that, in the case of positive dielectrophoresis, t he particles will form chainlike aggregates aligned parallel to the el ectric field lines and attracted towards the higher electric-field reg ion; whereas, for negative dielectrophoresis, the particles will form disklike aggregates aligned perpendicularly to the electric-field line s and rep-lied from the higher electric-field region. The theory also provides some insight regarding the dependence of the particle aggrega tion on the frequency or the applied nonuniform ac electric field. The predictions of the theory are consistent with the characteristic patt erns of cell aggregation observed previously in high-gradient electric -fields generated in microelectrode systems.