Mixing Enhancement by a Micro-Rotor in Step Microchannel

This paper describes numerically the design and performance of an active mixer which aims to exploit the chaos generated by a micro-rotor in continuous flow. The mixer consists of a step contraction-expansion microchannel and a micro-rotor placed at the step. The micro-rotor can be set into motion by laser power or magnetic field, inducing 3D motion in the surrounding fluid. By tracking streaklines from the inlet, we observed that fluids from the inlet can penetrate into the space between the paddles of the rotor, and then are mixed here. The streaklines also show that two fluids are twisted 90 degrees after passing the rotor region. It implies that mixing in the exit channel occurs in the height instead in the width of the exit channel. It makes the mixer applicable for channels with high aspect ratio of the cross section. The effectiveness mixing of the mixer is measured by the homogeneity distribution of a passive scalar on the outlet of the mixer. The results show that effectiveness of convective mixing induced by the rotor depends on Strouhal number, which is defined as the ratio of tip paddle velocity to mean flow in the channel. Mixing efficiency increases with increasing Strouhal number.