Erosion dynamics of a wet granular medium

Liquid may give strong cohesion properties to a granular medium, and confer a solid-like behavior. We study the erosion of a fixed circular aggregate of wet granular matter subjected to a flow of dry grains inside a half-filled rotating drum. During the rotation, the dry grains flow around the fixed obstacle. We show that its diameter decreases linearly with time for low liquid content, as wet grains are pulled-out of the aggregate. This erosion phenomenon is governed by the properties of the liquids. The erosion rate decreases exponentially with the surface tension while it depends on the viscosity to the power -1. We propose a model based on the force fluctuations arising inside the flow, explaining both dependencies: the capillary force acts as a threshold and the viscosity controls the erosion time scale. We also provide experiments using different flowing grains confirming our model

Crucial role of side walls for granular surface flows: consequences for the rheology

In this paper we study the steady uniform flows that develop when granular material is released from a hopper on top of a static pile in a channel. We more specifically focus on the role of side walls by carrying out experiments in setup of different widths, from narrow channels 20 particle diameters wide to channels 600 particle diameters wide. Results show that steady flows on pile are entirely controlled by side wall effects. A theoretical model, taking into account the wall friction and based on a simple local constitutive law recently proposed for other granular flow configurations (GDR MiDi 2004), gives predictions in quantitative agreement with the measurements. This result gives new insights in our understanding of free surface granular flows and strongly supports the relevance of the constitutive law proposed.Comment: a forgotten square root in Appendix B (Eq B4), and corrected coefficients in Appendix C; 25 pages, 17 figures, published in J. Fluid Mec

Comment on ''Measurement of Effective Temperatures in an Aging Colloidal Glass''

We measure the fluctuations of the position of a silica bead trapped by an optical tweezers during the aging of a Laponite suspension. We find that the effective temperature is equal to the bath temperature

Accretion Dynamics on Wet Granular Materials

Wet granular aggregates are common precursors of construction materials, food, and health care products. The physical mechanisms involved in the mixing of dry grains with a wet substrate are not well understood and difficult to control. Here, we study experimentally the accretion of dry grains on a wet granular substrate by measuring the growth dynamics of the wet aggregate. We show that this aggregate is fully saturated and its cohesion is ensured by the capillary depression at the air-liquid interface. The growth dynamics is controlled by the liquid fraction at the surface of the aggregate and exhibits two regimes. In the viscous regime, the growth dynamics is limited by the capillary-driven flow of liquid through the granular packing to the surface of the aggregate. In the capture regime, the capture probability depends on the availability of the liquid at the saturated interface, which is controlled by the hydrostatic depression in the material. We propose a model that rationalizes our observations and captures both dynamics based on the evolution of the capture probability with the hydrostatic depression

Hydrodynamic modeling of granular flows in a modified Couette cell

We present simulations of granular flows in a modified Couette cell, using a continuum model recently proposed for dense granular flows. Based on a friction coefficient, which depends on an inertial number, the model captures the positions of the wide shear bands. We show that a smooth transition in velocity-profile shape occurs when increasing the height of the granular material, leading to a differential rotation of the central part close to the surface. The numerical predictions are in qualitative agreement with previous experimental results. The model provides predictions for the increase of the shear bands width when increasing the rotation rate.Comment: 4 page

Intermittent flow in yield-stress fluids slows down chaotic mixing

In this article, we present experimental results of chaotic mixing of Newtonian uids and yield stress fluids using rod-stirring protocol with rotating vessel. We show how the mixing of yield stress fluids by chaotic advection is reduced compared to the mixing of Newtonian fluids and explain our results bringing to light the relevant mechanisms: the presence of fluid that only flows intermittently, a phenomenon enhanced by the yield stress, and the importance of the peripheral region. This finding is confirmed via numerical simulations. Anomalously slow mixing is observed when the synchronization of different stirring elements leads to the repetition of slow stretching for the same fluid particles.Comment: 5 page

Probability density functions of work and heat near the stochastic resonance of a colloidal particle

We study experimentally and theoretically the probability density functions of the injected and dissipated energy in a system of a colloidal particle trapped in a double well potential periodically modulated by an external perturbation. The work done by the external force and the dissipated energy are measured close to the stochastic resonance where the injected power is maximum. We show a good agreement between the probability density functions exactly computed from a Langevin dynamics and the measured ones. The probability density function of the work done on the particle satisfies the fluctuation theorem

Matériaux granulaires en transformation : dynamique et structure

De nombreux procédés industriels requièrent la transformation d'un matériau granulaire dit réactif en un produit fini. Au cours de tels processus, un couplage fort apparait entre la réponse de la microstructure granulaire et les transformations physico-chimiques qui affectent ses constituants. Nous étudions l'évolution d'un empilement granulaire fait de disques métalliques inertes au sein duquel sont placés des glaçons. Au cours de la fonte de ces grains réactifs, le milieu laisse apparaitre des réarrangements complexes tant spatialement que temporellement. Par traitement d'images, nous analysons ce processus de réorganisations en évaluant l'ampleur spatiale et temporelle des évènements ainsi qu'en caractérisant les modifications de la structure solide

Instabilité d'érosion d'une interface granulaire sèche-humide.

L’addition de liquide dans un milieu granulaire peut lui conférer une cohésion importante, et lui permettre de se comporter comme un substrat solide. Un milieu hétérogène contenant du liquide présentera donc plusieurs phases de natures différentes. L’interaction entre phase sèche et humide donne lieu à des échanges, arrachements de grains à la phase humide, ou agglomération de grains initialement secs. Ces échanges peuvent aboutir à des changements de morphologie des domaines. Nous avons mis en place une expérience d’érosion d’un tas humide cohésif dans une cellule mince, entre deux plaques de verre. L’écoulement de grains secs apportés à débit constant produit l’érosion du milieu humide par l’arrachement régulier de grains au tas. Nous avons pu observer la déstabilisation d’une interface initialement plane. Le couplage entre la contrainte de l’écoulement et la forme du tas crée des structures en forme de marches se propageant vers l’amont par l’érosion. Les propriétés de ces marches (taille, vitesse de propagation…) dépendent notamment de la pente du tas formé