Non-reciprocity across scales in active mixtures

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. In this talk, I will discuss how bacterial mixtures that interact via mediated interactions like quorum-sensing and chemotaxis are prototypical examples of nonequilibrium entities that interact via non-reciprocal interactions.

By explicity relating microscopic and macroscopic dynamics, I will show that non-reciprocity may fade as coarse-graining proceeds, leading to large-scale bona fide equilibrium descriptions. In turns, this allows accounting quantitatively, and without fitting parameters, for the rich behaviors observed in microscopic simulations including phase separation, demixing or multi-phase coexistence. I will 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 predicting the phase diagram of the system starting from its microscopic description. All in all, I will try to convey the idea that the fate of non-reciprocity across scales is a subtle and important question.