Irreversibility and chaos in active particle suspensions

We investigate the collective behavior of active suspensions of microswimmers immersed in a viscous fluid through numerical studies. We consider the two kind of organisms generally studied in experimental and theoretical studies, namely the pullers and the pushers, which differ in the way they are able to swim. We model them such that the body shape and the flagella used to swim are mathematical described. We tackle the problem using a new numerical approach, based on fictitious domains, which allows to fully solve the fluid-structure interaction and therefore exactly captures both the far- and near-field interactions between swimmers Considering a two-dimensional domain for simplicity, we first investigate the state of bacterial turbulence in an unbounded domain, analyzing also their dynamical properties in terms of chaos. Then we analyze the rheological properties of the suspension by varying the different parameters. Our main findings are the following: (i) At variance with some previous studies, we show that pullers are able to trigger bacterial turbulence, although there is an asymmetry between ellipsoidal puller and pusher swimmers when elongated. (ii) Moreover, spherical pullers and pushers are found to be indistinguishable from a macroscopic point of view. (iii) We show that the complex dynamics, in particular the difference between pushers and pullers in collective properties, is related to a spontaneous breaking of the time-reversal symmetry. The key ingredients for breaking the symmetry are the concentration of the suspension and the shape of the organisms. (iv) The suspensions are chaotic in all cases, but that plays no role with regard to the macroscopic properties of the suspension. (v) Rheological signatures, already known in experiments, are analyzed and explained within this framework. We unambiguously show that our results are due to hydrodynamic interactions, whereas collisions or contacts do not play a role. The present results should be taken into account for the proposition of simplified models.