Experimental Investigation on Flow and Mixing in Rotating Microfluidics

Mixing in rotational micro-chamber has been carried out both experimentally and numerically with intent of improving mixing in viscous dominated microfluidics. In experiment, an enclosed chamber made of machined PMMA cut-out with a PDMS cover was set-up on the rotational platform. Different geometries of the chamber (on the order of several mm) with constant height yet different angular span 5, 10, 15, 20 deg. and radial extents 1.5 and 3 mm were used in the tests. While the two different dyes each occupying half of the chamber took a long time (e.g. nearly 40 minutes for a 20-degree span chamber with inner and outer radii, respectively, 35 and 36.5 mm and height 0.5 mm) for mixing by molecular diffusion in the chamber, faster mixing (about 2.5 minute) can be achieved under continuous acceleration and deceleration rotation with a linear rate of 25 rad/s2. The time for mixing per unit volume (Specific Mixing Time - SMT) was determined experimentally as a function of geometry. The SMT increases with increasing vorticity strength as a result of increasing size of the rotating chamber and/or increasing the magnitude of the linear acceleration and deceleration rates. Mixing is also studied by numerical simulation of the tested geometry. Comparing with the experiments, we found good agreement for the SMT between test results and numerical model for small-angled chamber while the numerical model seems to under-estimate the SMT for large-angled chamber. Further, we have also verified qualitatively in the experiment the flow pattern of the primary vortex in the radial-azimuthal plane responsible, to a large extent, for mixing in the chamber. Two approaches have been taken to observe and confirm the primary vortex being set up by continuous acceleration-and-deceleration of the chamber. The first approach adopted color tracing with two miscible dyed liquids, and the second approach used neutrally buoyant particles to trace fluid particle flow in the rotating chamber.