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Stochastic Pairwise Forces Enhance Tracer Diffusion in Nonmotile Active Matter

Henry Alston1,2, Raphaël Voituriez3,4, and Thibault Bertrand1

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Phys. Rev. Lett. 136, 128303 – Published 27 March, 2026

DOI: https://doi.org/10.1103/yxch-hpbv

Abstract

Stochasticity is a defining feature of the pairwise forces governing interactions in biological systems—from molecular motors to cell-cell adhesion—yet its consequences on large-scale dynamics remain poorly understood. Here, we show that reciprocal but randomly fluctuating interactions between particles create active suspensions that can enhance the diffusion of an external tracer particle, even in the absence of self-propulsion or nonreciprocity. Starting from a lattice model with pairwise dynamics that minimally break detailed balance, we derive a coarse-grained dynamical theory for spatiotemporal density fluctuations and reveal an elevated effective temperature at short wavelengths. We then compute the self-diffusion coefficient of a tracer particle weakly coupled to our active fluid, demonstrating that purely reciprocal stochastic interactions provide a distinct and generic route to enhanced diffusivity in dense nonequilibrium suspensions.

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Phys. Rev. Lett. 136, 128303– Published 27 March, 2026
  • Received 27 August 2025
  • Accepted 5 February 2026
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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.

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