Non-reciprocal alignment induces asymmetric clustering in active mixtures

Abstract: Heterogeneity is ubiquitous in biological and synthetic active matter systems that are inherently out of equilibrium. Typically, such active mixtures involve not only conservative interactions between the constituents, but also non-reciprocal couplings, whose full consequences for the collective behavior still remain elusive. Here, we study a minimal active non-reciprocal mixture with both, symmetric isotropic and non-reciprocal polar interactions. By combining a hydrodynamic theory derived from microscopic equations and particle-based simulations, we provide a scale-bridging view on the rich dynamics that occur even in absence of oscillatory instabilities. We show, in particular, that non-reciprocal alignment alone induces asymmetrical clustering at otherwise fully symmetric parameters. These density inhomogeneities go beyond the typical band formation in Vicsek-like systems. Within the asymmetric clustering state, single-species clusters chase more dilute accumulations of the other species.