Many microfluidic applications involve chemical reactions. Most often, the flow is predominantly laminar, and without active or passive mixing enhancement the reaction time can be extremely long compared to the residence time. In this work we demonstrate the merits of the combination of flow pulsation and geometrical characteristics in enhancing mixing efficiency in microchannels. Mixing was studied by introducing a mixing index based on the gray level observed in a heterogeneous flow of pure water and water colored by rhodamine B. The effects of the injection geometry at the microchannel inlet and the use of pulsed flows with average Reynolds numbers between 0.8 and 2 were studied experimentally and numerically. It appeared that the mixing index increases with the nondimensional residence time (τ), which is inversely proportional to the Reynolds number. In addition, we show that the mixing efficiency depends strongly on the geometry of the intersection between the two fluids. Better mixing was achieved with sharp corners (arrowhead and T intersections) in all cases investigated. In pulsed flow, the mixing efficiency is shown to depend strongly on the ratio (β) between the peak amplitude and the mean flow rate. Optimal conditions for mixing in the microchannels are summarized as a function of Reynolds number Re, the ratio β, and the geometries.

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