Experimental analysis of particle and fluid motion in ac electrokinetics

An ac electric field is applied to induce particle and fluid motion in a wedge-shaped microchannel. Micron-resolution particle image velocimetry (mu-PIV) is used to determine spatially resolved particle velocity and fluid velocity fields. Under steady-state conditions, the particles experience a balance between dielectrophoretic forces induced by the nonuniform electric field and Stokes' drag forces due to viscous interactions with the fluid. The particle velocity is therefore different from the fluid velocity because of dielectrophoresis. A variant of mu-PIV, two-color mu-PIV, is developed and used to uniquely determine the fluid velocity from the observation of particles without a priori knowledge of the electrical properties. This technique is used to explore ac electrokinetically generated fluid motion. A series of voltage levels at fixed frequency are applied to the wedge-shaped electrodes. The dependency of fluid velocity on applied voltage at different regions in the flow is obtained by fitting power-law curves. This is used to determine the underlying physical phenomena associated with ac electrokinetics. We found that both electrothermal effects and ac electroosmosis are important for the current experimental conditions. However, the electrothermal effect is dominant in the bulk fluid.