The role of sharp turns in inertial particle focusing in the microfluidic labyrinth chip

We investigated inertial, size-based focusing of particles in the microfluidic labyrinth device which consists of several sharp turns in addition to circular loops. Experiments were conducted over a range of fluid Reynolds number 125 ≤ Re_f  ≤ 417 and particle sizes and the results were compared to focusing in spiral channels. At a given particle size, we observe that focusing occurs at a higher critical Dean number De^* in the labyrinth compared to spiral channels, except at Re_f = 417. Moreover, focusing occurs at higher De^* for larger particles in labyrinth, except at Re_f = 417. CFD simulations showed a dominant corner vortex at Re_f = 417 which could be the source of this deviation. In general, we find that the presence of a corner vortex increases the local radius of streamline curvature making particles to focus at a lower De^*. Additionally, we measured the focusing length L_f to test the hypothesis that sharp turns help in focusing smaller particles. Across all Re_f, L_f was found to vary inversely with particle size. At Re_f = 125, only the smaller 7 and 12 µm particles focused at a significantly lower L_f as compared to the spiral. At all other Re_f, we did not observe this reduction in L_f for any of the particle sizes tested.