Particle AC electrokinetics in planar interdigitated microelectrode geometry

Electrokinetic movement of dielectric colloidal particles in an electric field has been previously studied by individually examining the effect of electro- and dielectrophoresis, and electroosmotic flow. In this paper, we use computer simulations and experiments to investigate the combined and frequency-dependent effect of dielectrophoretic and electroosmotic forces on the particle motion in nonuniform electric field inside a suspending, low-conductivity medium. The developed computational electrohydrodynamic model, which accounts for electric double-layer effects, is predictive. Specifically, for the case of a fringing field created by a potential difference applied to interfacial microelectrodes having a periodic, interdigitated configuration, it predicts the formation of two quasi-equilibrium zones where all forces are exactly or nearly balanced, resulting in a slow motion or trapping of particles. Existence of such zones depends on the frequency of the applied electrical excitation, particle size, and the dielectric permittivities of particles and the medium. The experimental visualization of the trajectories of fluorescently labeled latex particles qualitatively agrees with model predictions. The experiments also confirm the existence of one of the predicted quasi-equilibrium zones. Confirmation of the second zone requires different experimental conditions.