Thèses

Mardi 29 Septembre 2020 à 14h00.

Étude d'assemblées denses de colloïdes auto-propulsés : Comportement collectif et propriétés rhéologiques

''In the last decades, active matter has stepped up from a fascination about mesmerizing animal collective movements to well-controlled experiments in the laboratory. Abiotic active systems have been used as a model to develop new knowledge in non-equilibrium physics and it has been done extensively in rather dilute systems. However, investigation in crowded conditions is still lacking especially in experiments. This leads to the key objective of this study: to perform an experimental investigation of active systems at high density and to relate the observation to our knowledge in glassy physics. Besides, by considering such a system as a new kind of active material, we also aim to investigate it via microrheology. In both cases, the main question is to examine how activity influences a system whose passive counterpart is already out of equilibrium.

Our experimental system is a monolayer of gold-platinum Janus colloids, which become self-propelled upon adding a solution of hydrogen peroxide (H2O2). The monolayer is slightly inclined to cause an in-plane density gradient. We characterize the activity level from the sedimentation length and define an effective temperature, which monotonically increases with H2O2 concentration. With this setup, we can investigate a full range of densities from dilute to ergodic supercooled and to nonergodic glass regime. We find that standard glassy physics describes well the active supercooled regime provided the replacement of the temperature by the effective one. However, beyond the glass transition, we find that relaxation responds nonmonotonically to activity. We observe a dramatic slowdown of the relaxation when particles are weakly self-propelled; followed by fasterrelaxation at high enough activity level. By analyzing correlation of displacement orientations, we propose that directed motion makes cage exploration less efficient and thus slows down cooperative relaxation comparing to a passive glass. We, therefore, name this phenomenon ”Deadlock from the Emergence of Active Directionality (DEAD)”.

We also start a numerical investigation using a model of active Brownian particles. Unlike in experiment, we can precisely fix the density in simulation. We find a motility induced phase separation at moderate densities and high propulsion forces. Apart from this, the simulation well agrees with our experimental result at least up to the supercooled regime. In this way, we can study the glassy regime where we expect the DEAD phenomenon and gain a better insight into the active glassy system.

We also investigate the rheology of our experimental system by letting a probe fall in the active sediment. We find that the advection of the colloids around the probe quantitatively agree with a Stokes flow. This leads to a possibility to extract an effective viscosity.

Directeurs de thèse : Cécile Cottin-Bizonne et Mathieu Leocmach

Membres du jury :
Mme AMON Axelle Maître de Conférences HDR Université de Rennes 1 Examinatrice
Mme COTTIN-BIZONNE Cécile Directrice de Recherche CNRS Lyon Directrice de thèse
M. DAUCHOT Olivier Directeur de Recherche CNRS Paris Rapporteur
M. LEOCMACH Mathieu Chargé de Recherche CNRS Lyon Co-Directeur de thèse
M. RODNEY David Professeur des Universités Université Lyon 1 Examinateur
Mme ZACCARELLI Emanuela Chercheure Sénior Sapienza Université de Rome - Italie Rapporteur

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