When a “resting” foam keeps moving: a new understanding of soft materials

What actually happens inside a foam when we stop manipulating it? While we might instinctively think that it “comes to a standstill,” the reality is more subtle: its bubbles continue to reorganize themselves, in a way that is invisible to the naked eye. This phenomenon applies to a wide range of materials—foams, emulsions, pastes, gels—whose internal structure continues to evolve even when at rest.

In this study, the researchers used an ultrastable liquid foam as a model for a disordered material. Using high-speed X-ray tomorheoscopy—an advanced synchrotron imaging technique—they were able to simultaneously track changes in mechanical stresses and the movements of bubbles in space and time, thereby gaining direct insight into the internal dynamics after the flow had stopped.

The results shed new light on the subject. Contrary to what is often observed in complex systems, where relaxation follows slow, time-spread laws, the researchers show here that the foam relaxes in a simple and rapid manner, following an exponential law. Even more surprising: the rate of this relaxation does not depend on how the foam was previously set in motion. It is directly linked to a local phenomenon, on the scale of a few bubbles, corresponding to individual rearrangements.

This result highlights a regime that has been little documented until now: stress relaxation does not rely on large-scale collective effects, but on a succession of independent local events. This discovery challenges certain assumptions about the behavior of amorphous materials and opens up new avenues for better understanding and predicting their evolution.

Ultimately, this research could help improve control over the mechanical properties of many materials used in industry (cosmetics, food processing, construction materials) or in technological applications. It also provides a renewed conceptual framework for analyzing the influence of mechanical history on the state and stability of these complex systems.

Researcher Contacts

Christophe RAUFASTE, Professor, Université Côte d’Azur – Nice Institute of Physics – INPHYNI (Université Côte d’Azur / CNRS) christophe.raufaste@univ-cotedazur.fr
Stéphane SANTUCCI, CNRS Research Director – Physics Laboratory at the ENS de Lyon – LPENSL (ENS de Lyon / CNRS) stephane.santucci@ens-lyon.fr
Benjamin DOLLET, CNRS Research Director – Interdisciplinary Physics Laboratory – LIPhy (Université Grenoble Alpes / CNRS) - benjamin.dollet@univ-grenoble-alpes.fr

References

PRL, https://doi.org/10.1103/3np8-ljj1
ArXiv, https://doi.org/10.48550/arXiv.2602.23690