AC EOF in a rectangular microannulus

Abstract

Electrokinetic flow under an alternating current electric field is numerically studied in a rectangular microannulus whose inner and outer surfaces may have different zeta potentials. The main parameters that affect the shape of the flow field are electrokinetic diameter, dimensionless frequency, surface charge ratio, aspect ratio, and hydraulic diameter. An increase in the electrokinetic diameter leads to a decrease in the effective electric double layer thickness, while the reverse is true for the Poiseuille number, maximum velocity near the wall, and the flow rate. Very low-frequency flow exhibits lid-like velocity profiles; on the contrary, very high-frequency flow represents thin oscillating layers close to the walls and a nearly stationary bulk fluid. Within the limit of higher-than-one excitation frequencies, the response of the liquid inside the electric double layer to the time-periodic electric field is almost immediate, while the rest needs a finite time to follow the instantaneous changes in the applied electric field. Instantaneous one- or two-direction flow can be produced, depending on the relative sign of the surface charges. For a fixed value of the electrokinetic diameter, the Poiseuille number increases and the flow rate decreases as a result of an increase in the hydraulic diameter.