Numerical investigation of mixing in microchannels with patterned grooves

Mixing in microchannels with patterned grooves was studied numerically by CFD simulations and particle tracking technique. Point location, velocity interpolation and a fourth-order adaptive Runge-Kutta integration scheme were applied in the particle tracking algorithms. Using these algorithms, Poincare maps were calculated from the 3D velocity field exported from a CFD package for microfluidics (MemCFD(TM)). For small aspect ratio (alpha = 0.05) grooves, the results showed that there was no significant irregularity in the Poincare map, and indicated little chaotic effect. For high aspect ratio (alpha = 0.30) grooves, the flow pattern became more jumbled, but there was no apparent evidence that indicated the flow was chaotic, for Reynolds numbers up to five. However, Poincare maps could still be used to evaluate the performance of this type of micro-mixer. The particle trajectories recorded in the Poincare maps were circular-like patterns. By counting the number of dots to form one circle in the Poincare maps, the length of the channel to enable one recirculation could be calculated. The results indicated that this length had an exponential relation to the aspect ratio of grooves, and it was independent of the flow velocity. The CFD simulation showed that the transverse motion could fold and stretch fluids to increase their interfacial area. The results showed that micro-mixers with patterned grooves caused rotation of the fluid streams. This rotation can reorient the folding in the depth direction, and can enhance passive mixing in microfluidic devices with shallow channels.