Transport phenomena shape and constantly reorganize materials at every scale, influencing their dispersion, segregation and clustering. Range expansions coupled with fluid flows are of great importance in understanding the organization and competition of microbial populations in liquid environments.

I will present laboratory experiments, combined with numerical modeling, focused on the collective dynamics of microorganisms undergoing division in the presence of a flow. We have recently created an extremely viscous medium that allows us to grow cells on a controlled liquid interface over macroscopic scales. I will show that an expanding population of microorganisms can itself generate a flow, leading to an accelerated propagation and fragmentation of the initial colony. Finally, I will show the mechanism at the origin of this metabolically generated flow and how it can lead to Rayleigh-Plateau like instabilities in these microbial populations.