Rapid multivortex mixing in an alternately formed contraction-expansion array microchannel.

We report a contraction-expansion array (CEA) microchannel for rapidly and homogeneously mixing different types of fluids by multivortex induced in alternately formed rectangular structures of the channel. Rapid mixing can be achieved in a topologically simple and easily fabricated CEA microchannel, employing a synergetic combination of two kinds of vortices: (1) expansion-vortices being induced by flow separation due to an abrupt change of cross-sectional area of the channel in its expansion region, and (2) Dean-vortices being induced by centrifugal forces acting on a cornering fluid through the channel. We experimentally and numerically investigated expansion- and Dean-vortices, and demonstrated rapid mixing of an aqueous solution containing fluorescein or human red blood cells (RBCs) at different flow rates corresponding to Reynolds number (Re) ranging from 7.2 to 43.0. Over 90% mixing efficiency at a channel length of 14.4 and 19.5 mm was achieved at Re ≥ 28.6 (fluorescein solution and deionized water) and Re ≥ 21.5 (RBC suspension and phosphate buffered saline), respectively. The proposed CEA channel is expected to be useful for a wide range of applications where particles, cells and reagents must be rapidly and homogeneously mixed in microchannels.