Homogenization approach to the behavior of suspensions of noncolloidal particles in yield stress fluids

The behavior of suspensions of rigid particles in a non-Newtonian fluid is studied in the framework of a nonlinear homogenization method. Estimates for the overall properties of the composite material are obtained. In the case of a Herschel-Bulkley suspending fluid, it is shown that the properties of a suspension with overall isotropy can be satisfactory modeled as that of a Herschel-Bulkley fluid with an exponent equal to that of the suspending fluid. Estimates for the yield stress and the consistency at large strain rate levels are proposed. These estimates compare well to both experimental data obtained by Mahaut et al [J. Rheol. 52, 287-313 (2008)] and to experimental data found in the literature

Shear-induced sedimentation in yield stress fluids

Stability of coarse particles against gravity is an important issue in dense suspensions (fresh concrete, foodstuff, etc.). On the one hand, it is known that they are stable at rest when the interstitial paste has a high enough yield stress; on the other hand, it is not yet possible to predict if a given material will remain homogeneous during a flow. Using MRI techniques, we study the time evolution of the particle volume fraction during the flows in a Couette geometry of model density-mismatched suspensions of noncolloidal particles in yield stress fluids. We observe that shear induces sedimentation of the particles in all systems, which are stable at rest. The sedimentation velocity is observed to increase with increasing shear rate and particle diameter, and to decrease with increasing yield stress of the interstitial fluid. At low shear rate ('plastic regime'), we show that this phenomenon can be modelled by considering that the interstitial fluid behaves like a viscous fluid -- of viscosity equal to the apparent viscosity of the sheared fluid -- in the direction orthogonal to shear. The behavior at higher shear rates, when viscous effects start to be important, is also discussed. We finally study the dependence of the sedimentation velocity on the particle volume fraction, and show that its modelling requires estimating the local shear rate in the interstitial fluid

Langues-U: A digital campus to increase students' autonomy?

International audienceThe aim of the digital campus Langues-U (http://www.langues-u.org) was to answer French universities' needs to prepare ESP students for the CLES (Certificat de Compétences en Langues de l'Enseignement Supérieur), providing the students with a self-study pedagogical tool. The online learning environment offers realistic activities and can be used by any learner working alone, either at home or in a language centre. In the form of blended learning the training can combine both distance learning and regular meetings with a teacher, according to the demands of the institutions deciding to implement it. It thus induces a modification of the roles of learner and teacher, and lets learners decide how they work and the resources they use according to their needs

Flows and heterogeneities with a vane tool: Magnetic resonance imaging measurements

We study the local flow properties of various materials in a vane-in-cup geometry. We use magnetic resonance imaging techniques to measure velocities and particle concentrations in flowing Newtonian fluid, yield stress fluid, and in a concentrated suspension of noncolloidal particles in a yield stress fluid. In the Newtonian fluid, we observe that the $\theta$-averaged strain rate component $d_{r,\theta}$ decreases as the inverse squared radius in the gap, in agreement with a Couette analogy. This allows direct comparison (without end-effect corrections) of the resistances to shear in vane and Couette geometries. Here, the mean shear stress in the vane-in-cup geometry is slightly lower than in a Couette cell of same dimensions, and a little higher than when the vane is embedded in an infinite medium. We also observe that the flow enters deeply the region between the blades, leading to significant extensional flow. In the yield stress fluid, in contrast with the usually accepted picture based on simulation results from the literature, we find that the layer of material that is sheared near the blades at low velocity is not cylindrical. There is thus a significant extensional component of shear that should be taken into account in the analysis. Finally and surprisingly, in the suspension, we observe that a thin non-cylindrical slip layer made of the pure interstitial yield stress fluid appears quickly at the interface between the sheared material and the material that moves as a rigid body between the blades. This feature can be attributed to the non-symmetric trajectories of the noncolloidal particles around the edges of the blades. This new important observation is in sharp contradiction with the common belief that the vane tool prevents slippage and may preclude the use of the vane tool for studying the flows of pasty materials with large particles

RHÉOLOGIE

National audienceLe Livre Blanc de la recherche en Mécanique, élaboré par le Haut Comité pour la Mécanique (HCM) dans le cadre de l’Association Française de Mécanique (AFM) vient pour rappeler son importance et montrer les nombreuses retombées de la Mécanique. .Les objectifs de ce Livre Blanc de la Recherche en Mécanique sont multiples : Identifier les besoins industriels, les technologies innovantes, Proposer des orientations de recherche et technologie pour faire initier des projets R&D associant le monde scientifique et les industriels qui vont contribuer à la création de la valeur permettant à nos industriels de gagner une part du marché, Apporter l’information pour des industriels, des décideurs mais aussi des scientifiques qui peuvent ignorer les ressources que recèle leur milieu. La communication entre ces grands acteurs en résulte naturellement.Le chapitre "Rhéologie" détaille les défis scientifiques propres à la discipline

Transport efficiency of metachronal waves in 3d cilia arrays immersed in a two-phase flow

The present work reports the formation and the characterization of antipleptic and symplectic metachronal waves in 3D cilia arrays immersed in a two-fluid environment, with a viscosity ratio of 20. A coupled lattice-Boltzmann-Immersed-Boundary solver is used. The periciliary layer is confined between the epithelial surface and the mucus. Its thickness is chosen such that the tips of the cilia can penetrate the mucus. A purely hydrodynamical feedback of the fluid is taken into account and a coupling parameter $\alpha$ is introduced allowing the tuning of both the direction of the wave propagation, and the strength of the fluid feedback. A comparative study of both antipleptic and symplectic waves, mapping a cilia inter-spacing ranging from 1.67 up to 5 cilia length, is performed by imposing the metachrony. Antipleptic waves are found to systematically outperform sympletic waves. They are shown to be more efficient for transporting and mixing the fluids, while spending less energy than symplectic, random, or synchronized motions

Evaluer les équipements des lieux de vie à l'aide de simulations réelles et virtuelles

International audienceLes recherches présentées visent à concevoir une méthode pour évaluer des objets réels ou virtuels qu'il n'est pas toujours possible de manipuler. Il faut notamment préciser les dimensions d'évaluation pertinentes en fonction de l'environnement réel ou virtuel, et de la tâche qu'il est possible d'effectuer : manipuler l'objet ou observer un tiers manipuler. Ces 2 tâches impliquent des perspectives différentes sur l'objet. Par ailleurs l'utilisateur peut adopter différents référentiels pour coder les caractéristiques spatiales des objets. La perspective et le référentiel spatial participent à la représentation mentale d'une scène, il s'agit alors d'observer leurs effets sur l'évaluation d'un objet. Des tests préliminaires ont permis d'explorer l'induction d'une perspective et valider un outil d'évaluation. L'étude qui a suivi visait à observer l'effet de la perspective d'un acteur ou d'un observateur sur l'évaluation de 4 objets, dans les dimensions risques, acceptabilité, utilisabilité et utilité. Une pré-analyse montre d'une part que les observateurs ont tendance a évalué les objets plus positivement que les acteurs, et d'autre part que les participants n'ont pas été en mesure d'évaluer le risque. Les analyses e

Rheological behaviour of suspensions of bubbles in yield stress fluids

The rheological properties of suspensions of bubbles in yield stress fluids are investigated through experiments on model systems made of monodisperse bubbles dispersed in concentrated emulsions. Thanks to this highly tunable system, the bubble size and the rheological properties of the suspending yield stress fluid are varied over a wide range. We show that the macroscopic response under shear of the suspensions depends on the gas volume fraction and the bubble stiffness in the suspending fluid. This relative stiffness can be quantified through capillary numbers comparing the capillary pressure to stress scales associated with the rheological properties of the suspending fluid. We demonstrate that those capillary numbers govern the decrease of the elastic and loss moduli, the absence of variation of the yield stress and the increase of the consistency with the gas volume fraction, for the investigated range of capillary numbers. Micro-mechanical estimates are consistent with the experimental data and provide insight on the experimental results.Comment: submitted to Journal of non Newtonian Fluid Mechanic

Coupling of elasticity to capillarity in soft aerated materials

We study the elastic properties of soft solids containing air bubbles. Contrary to standard porous materials, the softness of the matrix allows for a coupling of the matrix elasticity to surface tension forces brought in by the bubbles. Thanks to appropriate experiments on model systems, we show how the elastic response of the dispersions is governed by two dimensionless parameters: the gas volume fraction and a capillary number comparing the elasticity of the matrix to the stiffness of the bubbles. We also show that our experimental results are in good agreement with computations of the shear modulus through a micro-mechanical approach.Comment: submitted to Soft Matte