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  • Open Access

Nonreciprocal Mixtures in Suspension: The Role of Hydrodynamic Interactions

Giulia Pisegna1, Navdeep Rana1, Ramin Golestanian1,2,*, and Suropriya Saha1,†

  • *Contact author: ramin.golestanian@ds.mpg.de
  • †Contact author: suropriya.saha@ds.mpg.de
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Phys. Rev. Lett. 135, 108301 – Published 3 September, 2025

DOI: https://doi.org/10.1103/gbg1-lwwt

Abstract

The collective chasing dynamics of nonreciprocally coupled densities leads to stable traveling waves which can be mapped to a model for emergent flocking. In this Letter, we couple the nonreciprocal Cahn-Hilliard model to a fluid to minimally describe scalar active mixtures in a suspension, with the aim to explore the stability of the waves, i.e., the emergent flock in the presence of self-generated fluid flows. We show that the emergent polarity is linearly unstable to perturbations for a specific sign of the active stress recalling instabilities of orientational order in a fluid. Using numerical simulations, we find, however, that nonreciprocity stabilizes the waves against the linear instability in a large region of the phase space.

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Phys. Rev. Lett. 135, 108301– Published 3 September, 2025
  • Received 12 February 2025
  • Accepted 21 July 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

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