Numerical and experimental investigation on flow and mixing in batch-mode centrifugal microfluidics

The vortical flow leading to mixing in micro-chamber due to Euler/inertial force from continuous cyclic acceleration-and-deceleration under unidirectional rotation has been investigated both numerically and experimentally. The primary vortex as seen from numerical model during acceleration was confirmed by flow visualization by monitoring the motion of the interface between two miscible dyed liquids, and by tracking the movement of neutrally buoyant particles in the flow. In a typical test, a total of 15 cycles (equivalent to 150 s) was required to achieve uniform mixing by continuously angular acceleration-and- deceleration. This is much shorter when compared to mixing of liquids by molecular diffusion for the same test geometry (stationary) which took at least 2400 s to reach uniform mixture. For practical application, the effectiveness of mixing is quantified by a specific mixing time (SMT), which corresponds to the time for mixing quality, α, to reach 90% normalized by the volume of the mixture in the chamber. Lower SMT is indicative of more effective mixing. SMT was found to decrease with increasing (a) outer radius, (b) angular span, and (c) acceleration/deceleration as a result of stronger vortical flow. For small angular span (i.e. 5°) wherein mixing is slow, increasing acceleration from 17 to 34 rad/s² can compensate mixing, whereas for larger angular span (i.e. 20°) this is not as significant as mixing is already quite effective. Experimental measurements on SMT agree well with those of the numerical model.