Light-actuated electrothermal microfluidic flow for micro-mixing

Abstract

In this study, we introduce light-actuated electrothermal (ET) flow as a means for fluid mixing in lab-on-a-chip (LOC) systems. The study begins with physically understanding the light-induced heating of an electrode surface involved in the generation of the ET flow. Four electrodes are chosen arbitrarily, and the change in the light-actuated ET flow velocity over each of the electrodes is measured by particle image velocimetry (PIV). The selected electrodes contain the following: silver (Ag), gold (Au), aluminum (Al), and indium tin oxide (ITO). The PIV data are analyzed based on the Beer-Lambert law and the thermal properties of the electrodes. The analysis confirms that the light-induced heating occurs effectively on electrode materials with a high optical absorption rate and low thermal conductivity. The use of the PIV is extended to investigate the dependence of light-actuated ET flows on electrical and thermal parameters. The increase in the electric potential and light intensity causes a parabolic and linear change in the ET velocity, respectively. At AC frequencies below the charge relaxation frequency of the fluid used, the ET velocity exhibits a plateau except for the frequency region where electroosmosis occurs. Based on the understanding of the flow characteristics, the mixing of two aqueous solutions by the light-actuated ET flow is attempted. This demonstrates that the ET flow generated in the form of a toroidal vortex can significantly improve the mixing length and mixing efficiency of two different fluids in a microchannel.