Description of the thesis topic

When describing the mechanical response of a solid at equilibrium, one can rely on three principles: energy conservation, symmetry of the response tensor, and Galilean invariance. It has recently been established that when the solid is made up of particles that inject energy into the system by spinning, none of these principles is verified. The mechanical response of the system is then highly anomalous, with a solid that can expand when rotated, or shear when compressed. The case where the solid is made up of self-propelled particles has been less studied. We know that there are modes of spontaneous collective actuation, but we have no idea how such a solid would respond to mechanical stimulation.

The aim of this thesis is to probe the mechanical response of a dense suspension of self-propelled colloidal particles. This system has been extensively studied in the host team.

In the course of this thesis, the candidate will develop a magnetic microrheology apparatus from an already available prototype. The application of a magnetic field to a magnetic bead immersed in the sample will exert known stresses, and optical microscopy of the displacements of the active particles will give access to the dynamic response tensor. In this way, the candidate will be able to characterize the anomalous response of the system in the elastic and elasto-plastic domains, as a function of the stress, but also of the activity or density of the system.

This project requires a strong background in soft matter physics, statistical physics or mechanics, a taste for experimental challenges, and a willingness to learn image and data analysis.

This PhD thesis is fully funded by ANR and is part of a collaboration with experts in numerical simulations in Japan (possible research visits).

Work Context

The Institute for Light and Matter (Institut Lumière Matière - iLM) is a joint CNRS-University of Lyon 1 research unit located on the Lyon Tech La Doua campus.

With more than 300 employees, including a hundred doctoral students and post-docs, the iLM is a major player in physics and chemistry research in the Auvergne Rhône Alpes region, internationally recognized for the excellence of its research.

The continuum between fundamental research, responses to major societal challenges and innovation is at the heart of the unit's approach. The entire staff is committed to promoting excellence and ethical and responsible research.

Its scientists explore six major thematic fields:
- Materials, Energy, Photonics
- Soft Matter
- Nanosciences
- Optics and Ultrafast Dynamics
- Theory and Modeling
- Living Matter, Health, Environment.

The Liquids and Interfaces team comprises over 20 permanent researchers, faculty members and engineers, and around the same number of PhD and post-doctoral students. Its scientists are interested in the physics of soft and complex matter, from liquids to granular materials and active matter. In particular, the team has recognized skills in cutting-edge experimental development, which enable to link microscopic observations and macroscopic behavior.

Although mainly focused on the subject of their own thesis, the candidate will be in contact with all the research activities of the team and, more broadly, of the laboratory as a whole, that will provide a rich scientific environment. Moreover, this thesis is part of a wider project involving collaboration with collaborators in Paris, as well as in Osaka and Tokyo, Japan. These exchanges are funded by the CNRS and will enable the PhD student to spend a research period in Japan.

The start date is negotiable between September 1, 2025 and December 1, 2025.