The present work describes results of experimental and theoretical studies of the behavior of dilute suspensions subject to high gradient AC electric fields. When a spatially non-uniform electric field is applied to a suspension, in which the particles and the suspending fluid have different dielectric permittivities andlor conductivities, the particles migrate towards the regions of high or low electric field strengths depending on the sign of their polarization. This phenomenon, referred to as dielectrophoresis, has found numerous applications for the control and manipulation of the particle motions.

Presented in Chapter 3 are the results of studying the field -induced motion and segregation of particles in macro-scale. 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 along the channel cross section perpendicular to the streamlines of the main flow. Following the application of a high-gradient strong ac field (~ several ky/mm), the particles were found to move towards both the high-voltage and grounded electrodes and aggregate along their edges. 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).

Chapter 4 is devoted to the studies of the field-induced particle motions and their segregation in suspensions flowing through microfluidic devices. Experiments were conducted on microfluidics with electrodes of different sizes affanged parallel and perpendicular to the flow. The conditions under which the interparticle electric interactions do not affect the particle trapping in high-field strength regions were identified.

In order to evaluate the effects of gravity on the particle motions and their segregation, the experiments have been conducted in microgravity environment aboard the NASA research aircraft. Appendix A presents the Test Equipment Data Package for the microgravity experiments, the results of which are currently in the process of being evaluated and would require further investigation.