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Synthetic Quorum Sensing and Absorbing Phase Transitions in Colloidal Active Matter

Thibault Lefranc1,*, Alberto Dinelli2,3,*, Carla Fernández-Rico4, Roel P. A. Dullens5, Julien Tailleur6,†, and Denis Bartolo1,‡

  • *These authors contributed equally to this work.
  • †Contact author: jgt@mit.edu
  • ‡Contact author: denis.bartolo@ens-lyon.fr
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Phys. Rev. X 15, 031050 – Published 22 August, 2025

DOI: https://doi.org/10.1103/8csn-71jk

Abstract

Unlike biological active matter that constantly adapt to their environment, the motors of synthetic active particles are typically agnostic to their surroundings and merely operate at constant force. Here, we design colloidal active rods capable of modulating their inner activity in response to crowding, thereby enforcing a primitive form of quorum sensing interactions. Through experiments, simulations, and theory we elucidate the impact of these interactions on the phase behavior of isotropic active matter. We demonstrate that, when conditioned to density, motility regulation can either lead to an absorbing phase transition, where all particles freeze their dynamics, or to atypical phase separation, where flat interfaces supporting a net pressure drop are in mechanical equilibrium. Fully active and fully arrested particles can then form heterogeneous patterns ruled by the competition between quorum sensing and mechanical interactions. Beyond the specifics of motile colloids, we expect our findings to apply broadly to adaptive active matter assembled from living or synthetic units.

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Popular Summary

In natural systems, living organisms often adapt their behavior based on how crowded their environment is—a phenomenon known as quorum sensing. For example, people slow down in crowded spaces, and bacteria change activity depending on population density. However, most synthetic active materials, made of self-propelled particles, lack this ability. They move, but they do not respond to their surroundings, which limits the complexity of patterns they can form. In our study, we introduce a new kind of synthetic material that incorporates quorum sensing to overcome this limitation.

We design microscopic colloidal rods that are powered by an electric field and programmed to sense local density. When the rods are in low-density regions, they actively roll; when they encounter crowded areas, they shut off their inner engine and stop moving. This simple feedback mechanism leads to surprising behaviors. We observe two main effects: one where active and inactive regions coexist, and another where the entire system freezes, entering what physicists call an “absorbing state.” By combining laboratory experiments, computer simulations, and theoretical modeling, we show how this adaptive behavior emerges from interactions between particles and their environment.

Our findings provide a new design strategy for synthetic materials that can react to their surroundings, a step toward creating active matter that does not just move, but adapts.

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Phys. Rev. X 15, 031050– Published 22 August, 2025
  • Received 20 December 2024
  • Accepted 4 June 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.

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