A parametric study of electrothermal flow inside an AC EWOD droplet

Abstract

A fully coupled electrothermal flow model is developed to simulate the electrothermal flow inside a droplet under AC electro-wetting on dielectric (EWOD) with a needle–planar-electrode configuration. The variations of electrothermal flow velocity and droplet temperature with frequency are compared with a classical electrothermal model and a previous experimental study. The investigated frequency range is extended to include dielectrophoretic (DEP) force dominant regime, and differences of characteristics of velocity and temperature fields in the Coulomb and DEP force dominant regimes are discussed and compared. Effects of solution concentration and needle electrode position and size are also examined. The results show that peak frequency to achieve the maximum electrothermal flow, underestimated by the classical model is correctly predicted by the present fully coupled model. After peak frequency, the velocity decreases till the valley frequency where the minimum electrothermal flow can be achieved, and then increases again, and finally keeps almost unchanged. After the valley frequency, the rotation of internal flow changes from counterclockwise to clockwise. When frequency is smaller, the smaller the solution concentration, the larger the electrothermal flow, while when frequency is larger, the result is opposite. The peak frequency and its corresponding electrothermal flow velocity increase with the increase of solution concentration. The needle electrode size has no influence on peak frequency, but the maximum electrothermal flow velocity at the peak frequency increases with decreasing electrode size. The smaller is the distance of needle electrode from the substrate, the larger are the peak frequency and the corresponding maximum electrothermal flow velocity. The numerical model developed and the results obtained in this study are useful for the design of droplet based electrothermal flow mixer in microfluidics.