Nonlocal Granular Rheology: Role of Pressure and Anisotropy

We probe the secondary rheology of granular media, by imposing a main flow and immersing a vane-shaped probe into the slowly flowing granulate. The secondary rheology is then the relation between the exerted torque T and rotation rate \omega of our probe. In the absence of any main flow, the probe experiences a clear yield-stress, whereas for any finite flow rate, the yield stress disappears and the secondary rheology takes on the form of a double exponential relation between \omega and T. This secondary rheology does not only depend on the magnitude of T, but is anisotropic --- which we show by varying the relative orientation of the probe and main flow. By studying the depth dependence of the three characteristic torques that characterize the secondary rheology, we show that for counter flow, the dominant contribution is frictional like --- i.e., T and pressure are proportional for given \omega --- whereas for co flow, the situation is more complex. Our experiments thus reveal the crucial role of anisotropy for the rheology of granular media.Comment: 6 pages, 5 figure

Particle Diffusion in Slow Granular Bulk Flows

We probe the diffusive motion of particles in slowly sheared three dimensional granular suspensions. For sufficiently large strains, the particle dynamics exhibits diffusive Gaussian statistics, with the diffusivity proportional to the local strain rate - consistent with a local, quasi static picture. Surprisingly, the diffusivity is also inversely proportional to the depth of the particles within the flow - at the free surface, diffusivity is thus ill defined. We find that the crossover to Gaussian displacement statistics is governed by the same depth dependence, evidencing a non-trivial strain scale in three dimensional granular flows.Comment: 6 page

Flow-induced Agitations Create a Granular Fluid

We fluidize a granular medium through localized stirring and probe the mechanical response of quiescent regions far away from the main flow. In these regions the material behaves like a liquid: high-density probes sink, low-density probes float at the depth given by Archimedes' law, and drag forces on moving probes scale linearly with the velocity. The fluid-like character of the material is set by agitations generated in the stirred region, suggesting a non-local rheology: the relation between applied stress and observed strain rate in one location depends on the strain rate in another location

Rheology of sedimenting particle pastes

We study the local and global rheology of non-Brownian suspensions in a solvent that is not density-matched, leading to either creaming or sedimentation of the particles. Both local and global measurements show that the incomplete density matching leads to the appearance of a critical shear rate above which the suspension is homogenized by the flow, and below which sedimentation or creaming happens. We show that the value of the critical shear rate and its dependence on the experimental parameters are governed by a simple competition between the viscous and gravitational forces, and present a simple scaling model that agrees with the experimental results from different types of experiments (local and global) in different setups and systems

From Frictional to Viscous Behavior: Three Dimensional Imaging and Rheology of Gravitational Suspensions

We probe the three dimensional flow structure and rheology of gravitational (non-density matched) suspensions for a range of driving rates in a split-bottom geometry. We establish that for sufficiently slow flows, the suspension flows as if it were a dry granular medium, and confirm recent theoretical modeling on the rheology of split-bottom flows. For faster driving, the flow behavior is shown to be consistent with the rheological behavior predicted by the recently developed "inertial number approaches for suspension flows.Comment: 5 pages, 4 figures, accepted for Phys. Rev. E. (R

Transition vitreuse de nanoparticules magnétiques en interaction

This PhD work is an experimental study of glass transitions in colloidal dispersions of charged and magnetic nanoparticles (ferrofluids). - The colloidal glass transition, observed at high concentrations, leads to an amorphous solid state out of the thermodynamical equilibrium. The charged particles are then strongly interacting, in a potential where the electrostatic repulsions prevail. Thanks to the original properties of ferrofluids, the complete structural degrees of freedom of the particles are considered. Positional ones are probed using static and dynamical scattering techniques (X-rays and neutrons). Glassy dynamics (non diffusivity, aging and intermittency) are reported at nanometric lengthscales. When a magnetic field is applied, the structure of the dispersion becomes anisotropic, as well as the translational dynamics of the particles and its aging. The rotational dynamics of the nanoparticles are probed using magneto-induced birefringence measurements. The rotational dynamics drastically slow down from a volume fraction threshold which depends on the intensity of the repulsions between particles. Its aging is studied on long time scales. An effective age is introduced to unite aging properties at different concentrations.- At low temperatures, the frozen dispersion constitute an assembly of disordered giant spins, which present some analogies with atomic spin glasses. Using SQUID magnetometry, we study the orientational dynamics of these superspins. We use a method borrowed from spin glasses to extract a dynamical correlation length between superspins; its size increases during the aging.Cette thèse est une étude expérimentale des transitions vitreuses de dispersions de nanoparticules magnétiques chargées (ferrofluides). - La transition vitreuse colloïdale, observée à forte concentration, conduit à un solide amorphe hors de l'équilibre thermodynamique. Les particules chargées sont alors en forte interaction, dans un potentiel dominé par les répulsions électrostatiques. Grâce aux propriétés originales des ferrofluides, nous considérons tous les degrés de liberté structuraux des particules. Ceux de position sont sondés par des mesures statiques et dynamiques de diffusion de rayonnement (rayons X et neutrons). Une dynamique vitreuse (non diffusive, vieillissante et intermittente) est observée à l'échelle nanométrique. En présence d'un champ magnétique, la structure des dispersions devient anisotrope, ainsi que la dynamique de translation des particules et son vieillissement. La dynamique de rotation des nanoparticules est sondée par des mesures de biréfringence magnéto-induite. Celle-ci se gèle à partir d'une fraction volumique qui dépend de l'intensité des répulsions entre particules. Son vieillissement est étudié sur des échelles de temps longues. Un âge effectif est introduit pour unir les propriétés de vieillissement à différentes concentrations. - A basse température, la dispersion gelée constitue un ensemble désordonné de spins géants, qui présente des analogies avec les verres de spins atomiques. En utilisant un magnétomètre SQUID, nous étudions la dynamique d'orientation de ces spins géants. Nous utilisons une méthode empruntée aux verres de spins pour extraire une longueur de corrélation dynamique; sa taille augmente au cours du vieilissement

Transition vitreuse de nanoparticules magnétiques en interaction

Cette thèse est une étude expérimentale des transitions vitreuses de dispersions de nanoparticules magnétiques chargées (ferrofluides). A forte concentration les dispersions forment un verre colloïdal, hors de l'équilibre thermodynamique. Nous étudions la dynamique de translation (mesures de diffusion de rayonnement) et de rotation (mesures de biréfringence) des particules en forte interaction répulsive. Des dynamiques lentes et des phénomènes de vieillissement sont observés pour la dynamique de translation et de rotation. L'effet d'un champ magnétique sur la dynamique est considéré: la dynamique de translation est alors anisotrope. A basse température, les dispersions gelées se comportent comme un verre de spins atomiques dont les moments magnétiques sont géants. Nous montrons, par des mesures de magnétométrie, que le vieillissement de la dynamique des moments s'accompagne de la croissance d'une longueur de corrélation dynamiquePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF