Experimental study on inertial focusing pattern in asymmetric contraction–expansion array microchannel

The transition of particle equilibrium pattern was investigated in contraction–expansion array (CEA) microchannel. The experimental results showed that, in addition to ordinary trajectory 1 (a focusing band near the center of the channel), another novel trajectory 2 could appear on the side near cavity sidewall. The effects of cavity dimension, Reynolds number, and particle diameter on the inertial focusing pattern of the particles were explored in detail. The equilibrium position of trajectory 1 will shift from the geometric center of the channel to the wall with cavity due to the influence of secondary flow generated by CEA units. The particles of 20 μm and 15 μm showed ideal focusing, while the particles of 10 μm focused in a wild band. It showed that the asymmetric CEA units can be used to regulate the focusing position of particles. Because of the squeezing action of two vortexes at the entrance of the contraction section, the trajectory 2 can be stable once it appears. In addition, the location of trajectory 2 is less affected by particle diameters and Re. In the same structure, trajectory 2 is more likely to appear with small particle diameter and large Re. Finally, we summarized the critical Re of focusing pattern transition. When it is greater than this critical value, trajectory 1 and trajectory 2 will exist in the channels simultaneously. It could be used to guide the design of the CEA channels. This basic research could provide a guidance to the application of microfluidic particle manipulation technique.