Thèse

Mercredi 22 Juillet 2026 à 14h00.

Flows of Dense Suspensions of Silica Particles in Microfluidic Drums: From Colloidal to Granular Dynamics

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Directeur de thèse / thesis director :
Antoine BÉRUT et Loïc VANEL

Membres du jury / jury members :
Yoël FORTERRE. Directeur de recherche CNRS IUSTI, Aix-Marseille Université. Rapporteur
Laurence TALINI. Directrice de recherche CNRS, SVI, CNRS/Saint-Gobain. Rapportrice
Catherine BARENTIN. Professeure des universités, ILM, Lyon 1 Université. Examinatrice
Élisabeth LEMAIRE. Directrice de recherche CNRS, INPHYNI, Université Côte d'Azur. Examinatrice
Antoine BÉRUT. Maître de conférences, ILM, Lyon 1 Université. Directeur de thèse
Loïc VANEL. Professeur des universités, ILM, Lyon 1 Université. Directeur de thèse

Résumé / Abstract :
Dense suspensions of silica microparticles in water offer a model system to investigate flow dynamics at the colloidal-to-granular crossover. In this regime, particles are large enough to sediment under gravity and form dense piles, yet small enough for thermal fluctuations to remain relevant. When a pile is tilted above the athermal angle of repose, it displays gravity-driven avalanche-like flows. As the angle decreases below this threshold, however, the motion does not stop; instead, thermal fluctuations sustain a slow creep flow. These dynamics therefore cannot be captured by either a purely athermal granular description or a purely thermal colloidal description.

In this thesis, we study free-surface flows of such suspensions in water-filled microfluidic rotating drums. By imposing an initial inclination and measuring the time evolution of the pile angle, we probe the relaxation of sedimented piles in a pressure-imposed configuration. This system allows us to investigate how Brownian motion and gravitational loading shape flow onset, creep dynamics, and the final arrested state of the suspension.

First, we show that sedimented silica microparticle piles exhibit aging during rest: increasing the time interval between sedimentation and the imposed inclination delays flow onset and reduces flow velocity, with both quantities depending logarithmically on this interval. This effect is strongest in thermally activated creep, whereas faster gravity-driven avalanches tend to erase it. We further demonstrate that aging occurs without measurable compaction or sample degradation, and that it cannot be attributed to frictional contact aging, since the particles remain electrostatically stabilized and effectively contactless. This behavior is reminiscent of colloidal-glass aging, in which dynamics slows down as particles explore more stable configurations without necessarily producing detectable structural evolution.

Second, we investigate the angle of repose across the thermal-to-athermal crossover using particles of different sizes. For the smallest particles, thermal creep drives complete relaxation of the pile, leading to a vanishing angle of repose. For larger particles, the pile arrests at a finite angle that increases with gravitational Péclet number, while remaining below the minimal angle expected for athermal frictionless grains. These results are consistent with a phenomenological model combining glass and jamming transitions, and show that dense colloidal suspensions can sustain non-zero angles of repose under conditions where thermal and gravitational effects are both relevant.

Finally, we study the creep dynamics of bidisperse systems composed of particles with distinct relaxation times. We find an asymmetric behavior: adding a small fraction of large particles to small-particle piles only weakly affects relaxation, whereas adding small particles to large-particle piles accelerates the initial flow. At later times, the relaxation crosses over to a slower regime. These observations indicate that bidisperse sediments are not governed by an average particle size alone, but by the coexistence of populations with distinct thermal activities.

Overall, this thesis reports on the flow dynamics of sedimented silica microparticles in microfluidic drums, focusing on three cases where Brownian motion and gravitational loading compete: aging during rest, repose-angle selection, and creep in bidisperse sediments. It thereby provides a unified picture of how dense suspensions flow and arrest at the colloidal-to-granular crossover.

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