Non-reciprocal alignment can induce asymmetric clustering in active repulsive mixtures

It is now well established that non-reciprocal systems exhibit intriguing, novel dynamical phases, the characteristics of which are shaped by the type and degree of non-reciprocity [1-3]. Here, we study a paradigmatic model of non-reciprocal active matter, namely a binary mixture of motile particles with completely symmetric repulsive interactions and non-reciprocal alignment couplings [3]. Using a combination of hydrodynamic theory, linear stability analysis, and particle-based simulations, we find dynamical, asymmetrical clustering situations, in which weakly polarized clusters form out of only one of the two species. Importantly, these asymmetric clusters emerge even though the isotropic repulsive interactions do not distinguish one species. Instead, the clustering is driven solely by non-reciprocal orientational couplings. For systems with antagonistic (anti-)alignment couplings, the resulting single-species clusters move and chase more dilute accumulations of the other species. We present a full non-equilibrium phase diagram in the parameter space of inter-species coupling strengths and compare with particle-based simulations, highlighting the impact of non-reciprocity on various scales.

[1] Z. You et al., PNAS 117, 19767 (2020).

[2] M. Fruchart et al., Nature 592, 363 (2021).

[3] K. L. Kreienkamp and S. H. L. Klapp, New J. Phys. 24, 123009 (2022).