Clustering and flocking of repulsive chiral active particles with non-reciprocal couplings

Abstract

Recently, non-reciprocal systems have become a focus of growing interest. Examples occur in soft and active matter, but also in engineered quantum materials and neural (brain) networks. Here, we investigate the impact of non-reciprocity on the collective behavior of a system of (dry) chiral active matter. Specifically, we consider a mixture of "circle swimmers" with steric interactions and non-reciprocal alignment couplings. Based on hydrodynamic equations which we derive from a set of Langevin equations, we explore the interplay of non-reciprocity, finite size, and chirality. We first consider, as a reference, one-species systems with reciprocal couplings. Based on a linear stability analysis and numerical simulations, we here observe three different types of collective behavior, that is, flocking, motility-induced phase separation, and a combination of both. Turning then to a non-reciprocal system, we find that non-reciprocity can turn otherwise stationary instabilities into oscillatory ones, affect the relative orientation of flocks, and, crucially, change the general type of instability. This illustrates the drastic impact of non-reciprocity and chirality on the emergent collective dynamics of active matter systems, with potentially far-reaching biological implications.

Chiral active matter

“…A question that has attracted a significant interest in the past few years is how MIPS is affected by chirality (i.e., ω = 0). Both studies based on particle-based simulations [29,93] and on the analysis of continuum equations [50,94,95] found that circular swimming (in 2D) tends to hinder MIPS. Remarkably, however, chirality also leads to a new phenomenon which does not occur for ω = 0: the emergence of finite dynamical clusters which counter-rotate with respect to the surrounding gas [29,93].…”

“…These rollers show various phases such as a disordered gas phase, rotating flocks and also vortices where the particles have aster-like orientations. A very recent work considers CAPs with non-reciprocal interactions, showing how these can qualitatively alter the (linear) stability of the disordered state [50].…”