Particle segregation in suspensions subject to high-gradient ac electric fields

An experimental and theoretical study is described dealing with the dielect rophoretic motion of individual particles in a static as well as in a flowi ng suspension subject to high-gradient ac electric fields. The experiments were performed on very dilute suspensions of neutrally buoyant hollow ceram ic spheres in a specially designed device in which the electric-field lines and the dielectrophoretic force were along the plane perpendicular to the streamlines of the main flow. Upon application of a high-gradient field (si milar to several kV/mm) to a quiescent suspension, the particles were found to move away from the electrodes and then to concentrate above the grounde d electrodes, forming a distinct boundary between the clean fluid and the r emaining suspension. This same field, when applied to a flowing suspension, caused the particles to concentrate within thin stripes parallel to the fl ow above the grounded electrodes and to travel with the suspending fluid wi thin these stripes. The theoretical model for the particle motion included only the dielectrophoretic force and the viscous drag, and required no fitt ing parameters because the particle polarizability was calculated independe ntly by measuring the concentration dependence of the complex permittivity of the suspension in a spatially uniform electric field of low strength (si milar to several V/mm). The computed particle motions and pattern formation s were found to be in a good agreement with the experimental data. These re sults demonstrate that the expression for the dielectrophoretic force which employs the value of the particle polarization measured in fields of low s trength can be used for describing the particle motions in fields of high s trength. This approach enables one to model a broad range of electro-hydrod ynamic phenomena in suspensions irrespective of whether or not they are per fectly insulating or perfectly conducting. (C) 2000 American Institute of P hysics. [S0021-8979(00)04022-6].