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  • Featured in Physics
  • Open Access

Emergent Dynamics of Active Elastic Microbeams

Q. Martinet1,†, Y. I. Li1,†, A. Aubret2, E. Hannezo1, and J. Palacci1,*

  • *Contact author: jeremie.palacci@ist.ac.at
  • †These authors contributed equally to this work.
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Phys. Rev. X 15, 041017 – Published 31 October, 2025

DOI: https://doi.org/10.1103/rjk2-q2wh

Abstract

In equilibrium, the physical properties of matter are set by the interactions between the constituents. In contrast, the energy input of the individual components controls the behavior of synthetic or living active matter. Great progress has been made in understanding the emergent phenomena in active fluids, though their inability to resist shear forces hinders their practical use. This motivates the exploration of active solids as shape-shifting materials, yet, we lack controlled synthetic systems to devise active solids with unconventional properties. Here we build active elastic beams from dozens of active colloids and unveil complex emergent behaviors such as self-oscillations or persistent rotations. Developing tensile tests at the microscale, we show that the active beams are ultrasoft materials, with large (nonequilibrium) fluctuations. Combining experiments, theory, and stochastic inference, we show that the dynamics of the active beams can be mapped on different phase transitions which are tuned by boundary conditions. More quantitatively, we assess all relevant parameters by independent measurements or first-principles calculations, and find that our theoretical description agrees with the experimental observations. Our results demonstrate that the simple addition of activity to an elastic beam unveils novel physics and can inspire design strategies for active solids and functional microscopic machines.

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Active Matter Gets Solid

Published 31 October, 2025

Researchers have determined the mechanical properties of a tiny beam made of active particles, laying the groundwork for future micromachines.

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Popular Summary

Most materials in our daily lives respond only when forces are applied: metals bend and rubber stretches. Living systems are different because they constantly consume energy, which allows them to move, reshape, and adapt in ways that passive matter cannot. While researchers have made progress in understanding active matter in fluids, such as bacterial swarms or schools of fish, active solids—materials made of active components that can sustain stress—remain much less explored, even though they are important for creating dynamic or shape-shifting materials. In this study, we introduce a new way to build and study such active solids.

We assemble microscopic elastic beams from dozens of active colloids. These beams show striking behaviors: when freely suspended, they rotate persistently, while beams fixed at one end oscillate rhythmically. By directly measuring their mechanical softness and fluctuations, and comparing the results with theoretical models, we show that these dynamics emerge from simple physical principles and can be tuned by boundary conditions alone.

Our findings highlight how adding activity to elastic structures creates new classes of materials with lifelike motion. Beyond advancing our understanding of active matter, this approach opens avenues for designing synthetic active solids with programmable dynamics. Such systems could inspire the development of microscopic machines or adaptive materials capable of autonomous reshaping and functional motion.

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Phys. Rev. X 15, 041017– Published 31 October, 2025
  • Received 5 March 2025
  • Accepted 11 September 2025
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