During the past decade a spectacular increase has occurred in the use of alternating current dielectrophoretic phenomena for trapping, concentrating and sorting of particles, cells, microorganisms and biomolecules. Compared to other available techniques dielectrophoretic methods have been demonstrated to be particularly well suited for the manipulation of fine particles at the nano- and micro-scales because they suppress undesirable electrical effects such as electrolysis and are insensitive to particle surface properties. The paper discusses not the transport of individual separate particles but their transport after they have first undergone a separation into low- and high-concentrated phases due to the application of a sufficiently strong electric field. This approach takes advantage of electric-field-induced interparticle cohesions and high viscosity of concentrated suspension in order to transport groups of particles at relatively high speeds.

Recently, we proposed and experimentally demonstrated (APL 90, 154104, 2007) the dielectrophoretic transport of such aggregations by the appropriate time-periodic energizing or grounding of electrodes in order to set up a moving electrical field. This carries along the aggregations in a fashion similar to a conveyor belt. The particles of interest are non-conducting and negatively polarized compared to their surrounding fluid. Such particles are attracted to regions of minimum electrical intensity. Once arrived in such a region the particles, because of interparticle attractions, then tend to clump together and once clumped, can be moved together by the moving electrical field as an approximately a two-dimensional columnar or cylindrical bolus. The moving electrical -intensity - minima that drive this can be located within the body of the fluid - rather than against fluid boundaries - so as to allow fairly easy movement of the boluses. As the electric minima change in location, the boluses follow them; in this way they are conveyed in desired directions. In particular, the proposed method can be utilized for building large-scale microparticle structures by first forming relatively simple structures comprised of particles and then translating them collectively to a work area for final assembly. The high suspension viscosity in the bolus plus effects of electric-field-induced inter-particle attractions help to keep the particles together during the bolus movement. As a result, stretching deformations potentially caused by viscous shear and diffusive spreading are reduced.

We present a two-dimensional modeling of the above process and comparison with the theoretical model. The thermodynamic and electrical equations governing the system are briefly discussed and also the electrical forcing of the Stokes equations. Solution methods are discussed. Particle flux and fluid flow patterns are presented as are the relative roles of convection and diffusion, the effects of viscous shear, and the range of feasible speeds for the bolus migration.