Thèses

Vendredi 25 Juillet 2025 à 9h30.

Microrheolgy of dense active systems : from colloids to cells

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Directeur de thèse / thesis director :
Cécile Cottin-Bizonne,
Hélène Delanoë-Ayari,
Mathieu Leocmach

Membres du jury / jury members :
Aurélie Dupont - Rapportrice
Jean-François Rupprecht Rapporteur
Manouk Abkarian - Examinateur
Jean-Paul Rieu - Examinateur

Résumé / Abstract :
This thesis sets out to explore the rheological properties of dense active systems, focusing on two distinct model systems: epithelial cell monolayers and monolayers of synthetic self-propelled colloids (Janus particles). The goal of this thesis is to explore the mechanical response of those 2D systems to perturbations, using a unified experimental approach based on Stokes flow geometry, the flow around a circular obstacle.
For the Janus colloids, I developed a novel experimental setup to drag obstacles through a dense monolayer of self-propelled particles. This allowed for the systematic study of the induced velocity and density fields. A key finding is the front-back asymmetry of the flow and a non-linear compressibility of the sediment, evidenced by an accumulation of particles in front of the obstacle and a depletion downstream the obstacle that are non-antisymmetric. I introduce a cicatrization length, characterizing the spatial extent of the depletion. This length correlates better with the mechanical pressure than with the density or activity level alone. Remarkably, molecular dynamics simulations in collaboration with Gianmarco Spera and François Graner confirmed that this cicatrization length and its associated timescale depend solely on mechanical pressure, and not on activity or the velocity of the obstacle. These results position pressure as a unifying control parameter for the cicatrization of Janus particles.
In the epithelial cell monolayer system, the mechanical response to a circular obstacle was studied in two geometries: a band, where flow direction is imposed, and a racetrack, where flow is generated intrinsically by the cells. In this work, I developed image analysis tools to obtain individual cell properties such as shape, elongation and area based on the Cellpose algorithm. The racetrack geometry allows us to access fluctuations of the system over large time and space scale. We perturbed the monolayer with 4 different drugs (Nocodazole, CK666, Simvastatin, and Para-aminoblebbistatin) and revealed the distinct contributions of cytoskeletal components against the molecular motors on the spatial correlation of the flow field. An attempt to model the cell monolayer with an active fluid model was made in collaboration with Nathan Shourick, Ibhrahim Cheddadi and Pierre Saramito leading to preliminary results, but the comparison with the experimental system needs to be improved. However, it shows how the Stokes geometry could be discriminant when comparing models and experiments.
In both, the Janus colloids and the epithelial cell monolayers, we are not yet able to characterize the rheological response of the system in a quantitative way. However, the Stokes geometry has shown experimental results that challenge the current theoretical understanding of active matter. In the case of the Janus colloids, further experimental development are needed such as the measurement of the force exerted on the obstacle, to determine the rheological response of the system.
Together, these results establish new experimental tools for probing the rheology of active systems, bridging the gap between synthetic and biological active matter, and offer pathways to model and predict their collective behaviour under perturbations.

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