Irreversibility and Chaos in Active Particle Suspensions

TL;DR

This paper uses direct numerical simulations to explore how active particle suspensions exhibit irreversibility and chaos, highlighting the roles of particle shape and interactions in breaking time-reversal symmetry.

Contribution

It demonstrates that particle shape and interactions, rather than chaos, are key to symmetry breaking and complex behaviors in active suspensions.

Findings

Large particle numbers and non-spherical shapes break time-reversal symmetry.

Pullers exhibit collective motion with distinct characteristics from pushers.

Chaos is not essential for the observed complex features.

Abstract

Active matter has been the object of huge amount of research in recent years for its important fundamental and applicative properties. In this paper we investigate active suspensions of micro-swimmers through direct numerical simulation, so that no approximation is made at the continuous level other than the numerical one. We consider both pusher and puller organisms, with a spherical or ellipsoidal shape. We analyse the velocity and the characteristic scales for an homogeneous two-dimensional suspension and the effective viscosity under shear. We bring evidences that the complex features displayed are related to a spontaneous breaking of the time-reversal symmetry. We show that chaos is not a key ingredient, whereas a large enough number of interacting particles and a non-spherical shape are needed to break the symmetry and are therefore at the basis of the phenomenology. Our numerical study also shows that pullers display some collective motion, though with different characteristics from pushers.