DEP particle dynamics and the steady drag assumption

The interaction of fluid drag, dielectrophoretic forces, and Brownian motion on a nanoparticle's motion is studied using a microfluidic chip with interdigitated electrodes. The flow domain is a 11.6 mum deep by 350 mum wide channel with 23 mum wide electrodes located at the bottom surface of the channel and a glass top surface. The electrodes are covered by a thin layer of silicon dioxide to insulate them from the fluid medium, suppressing electrolysis and local Joule heating. Although these phenomena have been considered by other researchers, our experiments and modeling reveal it to be a considerably more complicated phenomenon than previously thought. Using an adapted micro Particle Image Velocimetry technique along with microscopic imaging, particle motion in three-dimensions is measured and compared to predicted results, showing not only the expected horizontal DEP retarding force but also a vertical force away from the electrodes. Further, because of the spatially varying nature of both the DEP force as well as the drag force, one of the main assumptions made in many previous DEP studies must be seriously questioned-whether steady low Reynolds number particle dynamics are insufficient to predict the particle behavior.