Controlled transport of passive beads by phototactic swimming microorganisms*

Passive particles immersed in an active bath of micro-swimmers, either artificial swimmers or living microorganisms, may be displaced due to the activity of the suspension. This enhanced motion can lead to rich phenomena such as aggregation or phase separation.

In our experimental work, we study how passive beads are moved by directionally swimming microorganisms. We add plastic beads of size 50 microns to a dense suspension of the microalga Chlamydomonas reinhardtii (size: 10 microns). The suspension is enclosed in a large PDMS well of size ~1 cm. When a high intensity light is shined on one side of the well, the algae perform negative phototaxis and swim away from the light, eventually accumulating at the opposite well boundary.  As their local concentration grows, the algae push away the passive beads. By varying the incoming light direction, we manage to create complex patterns of passive particles, and concentrate them  in well-defined locations.