Phototactic behavior is coupled with pulsatile flow features to reveal the advantages of pulsation for separating motile algae cells in a double Y-microchannel. The underlying mechanism is as follows: during half of the pulsation cycle, when the flow rate is low, the phototactic microswimmers are mainly redirected by the external stimulation (light); while, during the rest of the cycle, the flow effects become dominant and the microswimmers are driven toward the desired outlet. The results show that in the absence of light source, the pulsatile flow has no advantage over the steady flow for separation, and the microswimmers have no preference between the exit channels; the separation index (SI) is around 50%. However, when the light is on, SI increases to 65% and 75% in the steady and pulsatile flows, respectively. Although the experiments are conducted on the well-known model alga, Chlamydomonas reinhardtii, a numerical simulation based on a simple model demonstrates that the idea can be extended to other active particles stimulated by an attractive or repulsive external field. Thus, the potential applications can go beyond algae harvesting to the control and enhancement of separation processes without using any mechanical component or chemical substance.
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