Mixing Enhancement in Simple Geometry Microchannels

Abstract

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 three different simple channel intersection geometries (“⊢”, “Y”, and “T”) by means of computational fluid dynamics (CFD) as well as physically mixing two aqueous reagents. In these geometries, both inlet channel segments (upstream of the confluence) and the outlet channel segment (downstream from the confluence) are 200 μm wide by 120 μm 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 m2 s−1, typical of many BioMEMS applications, and vary the flow rates of the reagents such that the inlet time-averaged 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 both inlets at 90 and 180 degrees out of phase in all three geometries. While mixing is good in all six cases, we demonstrate that the best results occur when both inlets are pulsed 90 degrees out of phase in the “T” geometry. In all six cases, the interface is shown to stretch, retain two folds, 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 inlet pulsing is 180 degrees out of phase is of particular interest as the outflow is constant.