Numerical investigation of electroosmotic mixing in a contraction–expansion microchannel

Electroosmotic micromixing is generally a highly desirable technique in microanalysis systems due to its ability to mix fluids at the microliter level. Herein, a contraction-expansion microchannel is used to conduct a numerical study on the mixing caused by AC electroosmotic. We investigate the flow and concentration field distributions in the microchannel and observe two circulation flows and four rotating vortices generated in the expansion chamber by the influence of AC electric fields. The generation of the rotating vortex is highly dependent on the relative magnitudes of the electroosmotic force and inertia force. Higher inertial force prevents vortex formation and significantly reduces the mixing quality under higher inlet velocity. We also explore the influence of inlet velocity, frequency, and voltage on the vortex formation and mixing performance. The best mixing quality (0.997) is obtained at the inlet velocity of 0.2 mm/s with 0.1 V voltage in a 70 μm length-channel and the pressure drop is only 1.58 Pa. The current micromixer provides a favorable method of mixing fluid based on electroosmotic and could improve the mixing quality in microfluidic analysis devices.