FIG. 4.

Recovery ratio, separation ratio, and concentration factor at different output $Q_{\text{target}}\hspace{0.17em}:Q_{\text{waste}}$ ratios and flow rates. The definitions of the recovery ratio, separation ratio, and concentration factor are given by Eqs. (3)–(5), respectively. PS particles (a) and E. gracilis cells (b) suspended in PBS. The discrepancy between the measured and theoretically predicted concentration factors is due to the central flow's weak drag force in the outlet tube at 50 μl/min in the case of the output flow ratio of 1:9. PS particles (c) and E. gracilis cells (d) suspended in PBS. The optimum conditions are an output ratio of 1:4 and 500 μl/min. The reason why the recovery ratio and hence the concentration factor are reduced at a high flow rate of 1000 μl/min is that the efficiency of focusing the particles and cells into the central acoustic pressure node is decreased due to their reduced retention time in the microchannel.