Abstract

Dilute suspensions of microswimmers show intriguing collective phenomena such as bacterial turbulence, where they self-organise in large scale chaotic structures. In this talk I will discuss a more subtle collective effect arising at lower densities when, in average, the suspension is homogeneous and isotropic. In particular, I will focus on the diffusion of passive tracers immersed in the suspension and on the statistics of the fluid velocity, discussing their non-trivial density dependence. I will present analytical results obtained using a kinetic theory approach that is non-perturbative in the swimmer density. The theory is in quantitative agreement with lattice Boltzmann simulations and captures the asymmetry between pusher and puller swimmers far below the transition to turbulence.

The talk is based on PRL 119 , 028005 (2017). Collaborators: J Stenhammar, RW Nash, D Marenduzzo, A Morozov.