Light-driven collective phenomena in motile microalgae

Motile microalgae modify their environment by absorbing light, consuming and releasing chemical compounds and generating flows. Flows, light and chemicals in turn influence their motion, driving the formation of patterns at macroscopic scales.

First, I will show how suspensions of photophobic cells surrounded by light can become unstable due to shading interactions: cells seek shade behind each other, leading to the complete phase separation of the system within minutes. These highly non-local vision-based interactions are responsible for the selection of a finite wavelength during the process, and thus for the formation of a striking branching pattern that retracts slowly. Algae inside dense areas undergo significantly less photo-damages than in planktonic conditions, demonstrating that phototaxis can efficiently contribute to photoprotection through collective behaviors.

On the other hand, phototaxis can be harnessed to control dense suspensions. We used light to create active jets where all cells swim in the same direction, and showed both their spontaneously breaking into droplets or buckling into a zigzag structure, depending on the cells orientation. These instabilities are purely hydrodynamic in nature, as shown by a new continuum theory and simulations. These results should hold generically at low Reynolds number, consistently with extensive studies on driven colloids. By unifying this broad class of systems under a single framework, our model paves the way toward the fine experimental control of active suspensions.