Nonreciprocal chiral active matter: Collective behavior and thermodynamics

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Recently, systems with nonreciprocal couplings have become a focus of growing interest. Examples occur in a wide range of fields and on different length and time scales, from non-Hermitian quantum systems to bacterial suspensions and (active) colloids, predator-prey systems and social systems involving sensing and communication. Recent field-theoretical studies on soft non-reciprocal systems, which are intrinsically out of equilibrium, have revealed novel types of collective dynamics. However, a broader understanding of these phenomena on the particle level has not yet been developed. Here we propose to investigate a nonreciprocal system from (dry) active matter, that is, a mixture of chiral active particles with nonreciprocal alignment couplings. Chiral particles build an important subclass of active matter. We here include the effect of mutual repulsion, yielding non-trivial clustering already in the absence of alignment. Our preliminary work based on a mean-field hydrodynamical theory reveals different types of nonequilibrium behavior which can be tuned by the degree of nonreciprocity. In the present project we aim, first, at complementing and thereby testing the hydrodynamic predictions by particle-based simulations of the underlying Langevin equations, with a focus on the specific effects induced by nonreciprocity. This includes, in particular, the important case of “dynamical frustration" coupling where particle of species A want to align with those from B, while the latter want to anti-align with A. Under such conditions we expect, based on field-theoretical studies, new intriguing behavior such as the emergence of symmetry-broken, time-dependent states. The second main goal is to characterize the collective behavior in terms of quantities from stochastic thermodynamics, in particular, measures of time-symmetry breaking (connected to entropy production). Such measures, which are accessible by the noisy particle trajectories or fields, have recently proven to be useful already for "simple" active fluids in the vicinity of motility-induced phase separation and flocking. In the present systems, nonreciprocity acts as an additional source of non-equilibrium, making an investigation of thermodynamic notions even more interesting. Starting from the particle level we aim, as a longer perspective, at a comparison with corresponding quantities from a field-level approach.