Nonreciprocity as a Generic Mechanism for Demixing in Flocking Mixtures

TL;DR

This paper demonstrates that weak nonreciprocal alignment interactions in flocking mixtures can cause large-scale structure formation and demixing, even without repulsive forces, through combined simulations and continuum analysis.

Contribution

It reveals that nonreciprocity universally destabilizes ordered flocking phases, leading to novel demixed structures and chaotic dynamics in binary flocking models.

Findings

Nonreciprocal interactions induce large-scale structures in flocking mixtures.

Demixing occurs without repulsive interactions due to nonreciprocity.

Structures include traveling bands and polar clusters with chaotic behavior.

Abstract

We show that even weak nonreciprocal alignment leads to large-scale structure formation in flocking mixtures. By combining numerical simulations of a binary Vicsek model and the analysis of coarse-grained continuum equations, we demonstrate that nonreciprocity destabilizes the ordered phase formed by mutually aligning or anti-aligning species in a large part of the phase diagram. For aligning populations, this instability results in one species condensing in a single band that travels within a homogeneous liquid of the other species. When interactions are anti-aligning, both species self-assemble into polar clusters with large-scale chaotic dynamics. In both cases, the emergence of structures is accompanied by the demixing of the two species, despite the absence of repulsive interactions. Our theoretical analysis allows us to elucidate the origin of the instability, and show that it is generic to nonreciprocal flocks.