Active mixing in diverging microchannels

In this study, active mixing in diverging microchannels is investigated experimentally. Using a custom-built dynamic pressure generator as the actuation source, the influences of the half-angle of the diverging section and the phase difference of the two actuating pressures in mixing performance are explored. Two flow characteristics, hydrodynamic instability and fluid lamellae, are identified as the main mechanisms leading to mixing enhancement. Even at very low Reynolds numbers, flow instability occurs in large half-angle diverging micromixers. The resulting tendril structures provide extra stretching and folding of the interface, and the degree of mixing increases accordingly. Nevertheless, flow instability alone is insufficient. To achieve excellent mixing in the diverging microchannels, the presence of both mechanisms is necessary. While pulsatile pressures vary in-phase, incoming streams flow collaterally and there is no fluid lamella effect. The concentration field is highly inhomogeneous even though tendril structures appear near the throat. In contrast, flow remains quite stable under anti-phase actuation. Fluid flows reciprocally between the two upstream branches during more than three quarters of a cycle, and patterns a series of fluorescence bands. Mixing is limited to molecular diffusion and performs poorly. When the phase difference shifts away from 0 and π, mixing performance improves significantly. To obtain the best results, a phase difference between 0.25π and 0.5π (or 1.5π and 1.75π) and a diverging micromixer with a large half-angle are preferred.