Suspensions of yeast cells (Saccharomyces cerevisiae) were used as model systems to investigate the electrokinetic behaviour of colloidal particles subjected to travelling electric fields generated using microelectrodes. Measurements were made over the frequency range 1 kHz to 10 MHz and for suspending medium conductivities in the range 6-260 mS m-1. A theoretical model is developed to provide a good description of the dependence of the observed translational motion, termed travelling-wave dielectrophoresis (TWD), on the dielectric properties of the particle and suspending medium, on the size of the particle, and on the magnitude and frequency of the applied field. Unlike conventional dielectrophoresis, the TWD effect is found to be related to the imaginary, rather than to the real, component of the induced dipole moment. Dielectrophoresis and electrorotation measurements were made to provide a further understanding of the observed effects and to support the theoretical model.

The spatial variations of the magnitude and phase of the electric field produced by electrodes used in electrorotation studies have been computed, using the charge density method at 10 degrees intervals for one complete AC cycle of the applied voltages. These results, together with the development of a more general theoretical treatment than has hitherto been applied to this problem, enable the time-averaged rotational torque and dielectrophoretic force acting on a particle to be determined as a function of its location within the inter-electrode space. Correction factors that can be applied to electrorotation spectra to take account of the relative position, or change in position, of the test particle are provided for electrodes of polynomial and circular geometry.