Odd collective phenomena in light-driven microalgae - Isabelle Eisenmann (University of Amsterdam, the Netherlands)

For motile microalgae, light is both a source of energy and a cue to orient in space. Their response to light can teach us about internal mechanisms, such as photosynthesis and photoprotection. On the other hand, physicists see them as active particles controllable by light, and use them to explore new active matter phenomena.

In the first part of this talk, I will briefly describe several collective phenomena that emerge in suspensions of swimming microalgae - mediated by shading, hydrodynamic or chemical interactions, and discuss the biological insights they can provide.

I will then describe recent results on odd active matter, using a novel experimental system of spinning microalgae. The cells self-assemble into a percolating chiral fluid as their density increases. Strikingly, microscopic and macroscopic vorticity have opposite signs: while individual cells spin clockwise, clusters orbit counterclockwise. Combining experiments and theory, we show that this chirality inversion is a direct consequence of the particles’ active (i.e. internally driven) nature. We then investigate odd transport on this percolating network. These experiments show how a living chiral fluid can behave fundamentally differently from an externally driven one.

Contact: Salima Rafaï