Irreversibility in scalar active turbulence: The role of topological defects

TL;DR

This paper investigates the origin of irreversibility in scalar active turbulence, showing that topological defects and their flow symmetries are key to understanding the deviation from reversible dynamics.

Contribution

It reveals that singularities in active stress and defect configurations primarily drive irreversibility in active turbulence, a novel insight into nonequilibrium behavior.

Findings

Irreversibility is linked to singularities in active stress.

Flow symmetries around defects determine irreversibility.

Specific defect pair configurations dominate irreversibility.

Abstract

In many active systems, swimmers collectively stir the surrounding fluid to stabilize some self-sustained vortices. The resulting nonequilibrium state is often referred to as active turbulence, by analogy with the turbulence of passive fluids under external stirring. Although active turbulence clearly operates far from equilibrium, it can be challenging to pinpoint which emergent features primarily control the deviation from an equilibrium reversible dynamics. Here, we reveal that dynamical irreversibility essentially stems from singularities in the active stress. Specifically, considering the coupled dynamics of the swimmer density and the stream function, we demonstrate that the symmetries of vortical flows around defects determine the overall irreversibility. Our detailed analysis leads to identifying specific configurations of defect pairs as the dominant contribution to irreversibility.