Novel low Reynolds number mixers for microfluidic applications

We present two new designs of compact mixers that can provide good mixing at low Reynolds numbers encountered in many microfluidic devices. The new designs benefit from curvature induced cross-stream vortices to enhance mixing of two co-flowing streams of fluids arranged side by side. One of the designs is a spiral of rectangular cross-section, while the other is a series of concentric circular channels arranged as a labyrinth. Both utilize the formation of sustained secondary flows to enhance mixing between two streams. Currently, the devices are fabricated in aluminum using standard machining techniques. However, they can be reduced further in size using standard microfabrication techniques. Mixing experiments were conducted in these channels at a Reynolds number of 6.8 using two sucrose solutions, one of which was laced with Rhodamine 6G dye. Compared to a experiment in an equivalent straight channel, a significant enhancement in the mixing of the two streams, as indicated by the intensity of the second fluid's color, was observed. The present designs provide a compact and easy-to-fabricate alternative to various other concepts proposed in literature.