Hydrodynamic Theory of Two-dimensional Chiral Malthusian Flocks

Leiming Chen; Chiu Fan Lee; John Toner

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arXiv · 2507.17762

Hydrodynamic Theory of Two-dimensional Chiral Malthusian Flocks

Leiming Chen·Chiu Fan Lee·John Toner

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Abstract

We study the hydrodynamic behavior of two-dimensional chiral dry Malthusian flocks; that is, chiral polar-ordered active matter with neither number nor momentum conservation. We show that, in the absence of fluctuations, such systems generically form a ``time cholesteric", in which the velocity of the entire system rotates uniformly at a fixed frequency b. Fluctuations about this state belong to the universality class of (2+1)-Kardar-Parisi-Zhang (KPZ) equation, which implies short-ranged orientational order in the hydrodynamic limit. We then show that, in the limit of weak chirality, the hydrodynamics of a system with reasonable size is expected to governed by the linear regime of the KPZ equation, exhibiting quasi-long-ranged orientational order. Our predictions for the velocity and number density correlations are testable in both simulations and experiments.

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Leiming Chen, Chiu Fan Lee, John Toner. 2025-07-08. Hydrodynamic Theory of Two-dimensional Chiral Malthusian Flocks. https://arxiv.org/abs/2507.17762

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