Non-reciprocity across scales in active mixtures

TL;DR

This paper investigates how non-reciprocal interactions in active bacterial mixtures evolve across scales, revealing conditions under which they diminish or persist, and providing a framework to predict complex behaviors from microscopic dynamics.

Contribution

It introduces a theoretical framework linking microscopic non-reciprocal interactions to macroscopic behaviors, showing how non-reciprocity can fade or survive coarse-graining in active matter.

Findings

Non-reciprocity may fade at large scales, leading to equilibrium descriptions.

Explicit coarse-graining predicts phase diagrams from microscopic interactions.

Conditions are derived for non-reciprocity to persist across scales.

Abstract

In active matter, the lack of momentum conservation makes non-reciprocal interactions the rule rather than the exception. They lead to a rich set of emerging behaviors that are hard to account for and to predict starting from the microscopic scale, due to the absence of a generic theoretical framework out of equilibrium. Here we consider bacterial mixtures that interact via mediated, non-reciprocal interactions like quorum-sensing and chemotaxis. By explicity relating microscopic and macroscopic dynamics, we show that non-reciprocity may fade as coarse-graining proceeds, leading to large-scale bona fide equilibrium descriptions. In turns, this allows us to account quantitatively, and without fitting parameters, for the rich behaviors observed in microscopic simulations including phase separation, demixing or multi-phase coexistence. We also derive the condition under which non-reciprocity is strong enough to survive coarse-graining, leading to a wealth of dynamical patterns. Again, the explicit coarse-graining of the dynamics allows us to predict the phase diagram of the system starting from its microscopic description. All in all, our work demonstrates that the fate of non-reciprocity across scales is a subtle and important question.