Positive dielectrophoresis and heterogeneous aggregation in high-gradient ac electric fields

Experiments were conducted in a parallel-plate channel in which an extremely dilute suspension of heavy, positively polarized spheres was exposed to an ac electric field under conditions such that the field lines were arranged in the channel cross section perpendicular to the streamlines of the main flow. To reduce the effects of the gravitational settling of the particles, the channel was slowly rotated around a horizontal axis. Following the application of a high-gradient strong ac field (∼ several kV/mm), the particles were found to move towards both the high-voltage and grounded electrodes and to form arrays of "bristles" along their edges. The process was also modeled theoretically by computing the trajectories of individual particles under the action of dielectrophoretic, viscous, and gravitational forces and under conditions of negligibly small particle Reynolds numbers. The model calculations required no fitting parameters because the particle polarizability was determined independently by measuring the frequency and concentration dependence of the complex dielectric permittivity of a suspension in a low-strength field (∼ V/mm). The predictions of this model were found to be consistent with the experimental data for the rate of particle accumulation on the electrodes but not for the aggregation pattern which, even for initially extremely dilute suspensions, appeared to be governed by the interparticle interactions and to be created by a two-step mechanism. The results of our studies provide the basic characteristics of the field-induced particle motions and segregation needed for the design and optimization of electrohydrodynamic apparatuses.