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

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

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

Elastic modulus of a colloidal suspension of rigid spheres at rest

International audienceBy modeling a colloidal suspension at rest as a solid, a new expression for the linear elastic modulus is obtained. This estimate is valid for a yield stress colloidal suspension submitted to a small strain. Interestingly, it is also possible to construct an hypothesis allowing one to recover the high-frequency modulus classically found by means of a classical ‘fluid approach'. However, in most of the situations, the moduli obtained by the two approaches are different

Editorial Viscoplastic fluids: From theory to application 2013

International audienceq This issue of the Journal of Non-Newtonian Fluid Mechanics includes a series of papers based on work presented at the international workshop on Viscoplastic fluids: from theory to application, held Nov. 18–21, 2013 in Rueil Malmaison, France. A list of participants is provided in Table 1. This was the fifth biannual meeting on this subject. The previous meetings were held in Banff (Alberta, Canada), Monte Verita (Ascona, Switzerland), Limassol (Cyprus) and Rio de Janeiro (Brasil) [1]. Like previous editions, the aim of the workshop was to bring together leading researchers in the field of viscoplastic fluids across several disciplines to foster the awareness and the transfer of ideas, both from academic research and industry. The program consisted in a single technical session and three invited keynote lectures. A total of 49 talks and 9 posters were presented, spanning fascinating topics from the coating of a viscoplastic fluid on a plate to the numerical simulation of the transition of viscoplastic fluid flows to turbulence. The workshop enjoyed an unprecedented number of 80 participants (Fig. 1), a popularity that emphasizes that viscoplastic fluids are a ''hot'' topic in the field of non-Newtonian Fluid Mechanics. Invited speakers were John Tsamopoulos (University of Patras, Greece), Guillaume Ovarlez (Laboratoire Navier, Université Paris-Est, France) and Fabrice Toussaint (Lafarge Centre de Recherche, France). John Tsamopoulos opened the meeting with an invited talk on the numerical simulation of yield stress fluid flows and its applications to the problem of the rising of a single bubble. He both elaborated on the technical details of the numerical tools available nowadays for the simulation of this class of flows and provided insight on the effect of elasticity and pressure oscillations. On the second day of the workshop, Guillaume Ovarlez gave an overview on the rheological behavior of suspen-sions of particles and bubbles in a yield stress fluid, illustrating his findings with a large number of experimental results. On the third day, Fabrice Toussaint bridged academic research and industrial concerns in the field of concrete rheometry, ranging from the fundamental behavior of concrete slurries and their characterization in rheometers to the fascinating last i-Phone application designed to tell the truck driver how quick the concrete slurry properties are evolving in the truck's rotating drum while driving to the construction site. Standard talks ranged from experiments, theory to numerical simulations, with an emphasis on the rheolo-gical behavior of carbopol gels and the more general question of how to experimentally characterize yield stress materials with thixotropy, a question that has been mobilizing the attention of the yield stress community for a few decades. The realistic and accurate modelling of the behavior of viscoplastic and thixotropic materials still remains an unsolved question in the field, as already underlined two years before in Rio de Janeiro, but also progress has been made, with new measurement techniques as, e.g., low amplitude oscillatory shear (LAOS). In particular, the workshop featured intense discussions on thixotropy and its mathematical modelling. Efforts in designing new numerical approaches with enhanced accuracy and fast convergence have seemed to slow down and the workshop was an occasion to collectively acknowledge that this research direction should be revived. The workshop took place at IFPEN-Rueil Malmaison, in the suburb of Paris, France. IFPEN is a large national research center in the field of energy, with nice facilities and well designed services. The social program started with an ice-breaking cocktail in a French-style coffee shop named ''Café Leffe'' in the heart of Rueil Malmaison, which was a casual occasion to chat with long-term colleagues and enables new participants to VPF to immerse into the yield (no) stress community and yield to the pleasure of enjoying French wines and the warm and friendly atmosphere o

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

On the collapse pressure of armored bubbles and drops

International audienceDrops and bubbles wrapped in dense monolayers of hydrophobic particles are known to sustain a significant decrease of their internal pressure. Through dedicated experiments we investigate the collapse behavior of such armored water drops as a function of the particle-to-drop size ratio in the range 0.02-0.2. We show that this parameter controls the behavior of the armor during the deflation: at small size ratios the drop shrinkage proceeds through the soft crumpling of the monolayer, at intermediate ratios the drop becomes faceted, and for the largest studied ratios the armor behaves like a granular arch. The results show that each of the three morphological regimes is characterized by an increasing magnitude of the collapse pressure. This increase is qualitatively modeled thanks to a mechanism involving out-of-plane deformations and particle disentanglement in the armor

The collapse of water drops protected by a granular armor

Les bulles et les gouttes protégées par une monocouche dense de particules hydrophobes peuvent résister mécaniquement à une baisse, éventuellement importante, de leur pression interne. Nous réalisons des mesures de pression d'effondrement sur ce type de gouttes en armure particulaire, pour des rapports de taille particule/goutte allant de 0,02 à 0,2. Nous montrons que ce rapport contrôle le comportement de la goutte lors de son « dégonflement » : pour de petits ratios la monocouche se froisse, pour des ratios intermédiaires elle se facette et, pour les rapports les plus grands, elle se comporte comme une coque granulaire. Les résultats montrent que chacun de ces régimes morphologiques est associé à un niveau de résistance maximale de la goutte lorsque sa pression interne est abaissée. Une modélisation purement géométrique parvient à rendre compte, au moins qualitativement, de ce comportement