Enhancement of microfluidic mixing using time pulsing

Many microfluidic applications require the mixing of reagents, but efficient mixing in these laminar (i.e., low Reynolds number) systems is typically difficult. Instead of using complex geometries and/or relatively long channels, we demonstrate the merits of flow rate time dependency through periodic forcing. We illustrate the technique by studying mixing in a simple "T" channel intersection by means of computational fluid dynamics (CFD) as well as physically mixing two aqueous reagents. The "T" geometry selected consists of two inlet channel segments merging at 90 degrees to each other, the outlet segment being an extension of one of the inlet segments. All channel segments are 200 mum wide by 120 mum deep, a practical scale for mass-produced disposable devices. The flow rate and average velocity after the confluence of the two reagents are 48 nl s(-1) and 2 mm s(-1) respectively, which, for aqueous solutions at room temperature, corresponds to a Reynolds number of 0.3. We use a mass diffusion constant of 10(-10) m(2) s(-1), typical of many BioMEMS applications, and vary the flow rates of the reagents such that the average flow rate remains unchanged but the instantaneous flow rate is sinusoidal ( with a DC bias) with respect to time. We analyze the effect of pulsing the flow rate in one inlet only as well as in the two inlets, and demonstrate that the best results occur when both inlets are pulsed out of phase. In this case, the interface is shown to stretch, retain one fold, and sweep through the confluence zone, leading to good mixing within 2 mm downstream of the confluence, i.e. about 1 s of contact. From a practical viewpoint, the case where the inlets are 180 degrees out of phase is of particular interest as the outflow is constant.