Irreversibility and chaos in active particle suspensions

Sergio Chibbaro, Astrid Decoene, Sebastien Martin, and Fabien Vergnet
Phys. Rev. Fluids 6, 013104 – Published 26 January 2021

Abstract

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.

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  • Received 7 June 2020
  • Accepted 7 January 2021

DOI:https://doi.org/10.1103/PhysRevFluids.6.013104

©2021 American Physical Society

Physics Subject Headings (PhySH)

Statistical Physics & ThermodynamicsNonlinear DynamicsPolymers & Soft Matter

Authors & Affiliations

Sergio Chibbaro

  • Sorbonne Université, Centre National de la Recherche Scientifique, UMR 7190, Institut Jean Le Rond d'Alembert, F-75005 Paris, France

Astrid Decoene*

  • Université Paris Sud, Laboratoire de mathématiques d'Orsay (CNRS-UMR 8628), Bâtiment 425, 91405 Orsay cedex, France

Sebastien Martin

  • Université Paris Descartes, Laboratoire MAP5 (CNRS UMR 8145), 75270 Paris cedex 06, France

Fabien Vergnet

  • Université Paris Sud, Laboratoire de mathématiques d'Orsay (CNRS-UMR 8628), Bâtiment 425, 91405 Orsay cedex, France

  • *astrid.decoene@math.orsay.fr

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Vol. 6, Iss. 1 — January 2021

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