MSC Seminar. 03/02/2025. Carine Douarche (Fast, Université Paris Saclay): “Instability in bacterial suspensions”

Bacteria are simple living beings endowed, for some of them, with long flagellar filaments that are considered as motility organs. Provided they find a microscopic energy source, they use flagella rotation to self-propel in the medium at low Reynolds number. Planktonic bacteria such as Escherichia coli exhibit a “run and tumble” motility pattern that results to a large-scale diffusion. While increasing the density in swimming bacteria, however, the suspensions display remarkable properties such as the appearance of collective motions. This type of motion is characterized by a local bacterial ordering associated with a characteristic correlation length scale, reminiscent of turbulent phenomena. The presence of active bacteria in the fluid confers also unconventional macroscopic properties to the suspension such as a decrease of the viscosity under low shear stress. The apparent viscosity of the active suspension decreases linearly with the bacterial density to reach zero with the onset of collective motions. With this active fluid of interesting rheological properties, we can thus revisit hydrodynamical instabilities: we study the active viscous Saffman-Taylor fingering depending on the bacteria concentration and the shear rate of the flow.