Papers by Abdoulaye Ndao Fall
Physical Review Research, 2022
We show that foams and emulsions can display a fundamentally different normal response to a simpl... more We show that foams and emulsions can display a fundamentally different normal response to a simple shear deformation. While foams dilate or push outwards on the shearing surfaces, known as a positive Poynting effect, in emulsions the Poynting effect can have either sign and can be tuned by changing the emulsion properties. We relate the sign of the Poynting effect to the presence of a compressible contact network supported by adhesive contacts. When the concentration of surfactant in the continuous phase is low, the emulsions are nonadhesive and push outward on their shearing surfaces, as do the foams. When the surfactant concentration is increased, the emulsions become adhesive due to depletion interactions, and the Poynting effect changes sign. We argue that the adhesive contact network develops a shear modulus that stiffens in response to dilation, which leads to the negative Poynting effect.
Multi-phase flows encountered in nature or in industry, exhibit non trivial rheological propertie... more Multi-phase flows encountered in nature or in industry, exhibit non trivial rheological properties, that can be understood better thanks to model materials and appropriate rheometers. Here, we use model unsaturated granular materials: assemblies of frictional spherical particles bonded by a small quantity of a wetting liquid, over a rough inclined plane. Our results show steady uniform flows for a wide range of parameters (the inclination angle and the mass flow-rate). A theoretical model, based on the Mohr-Coulomb yield criterion extended to inertial flows: = + () , in which and () are the cohesion stress and the internal friction coefficient respectively, gives predictions in quantitative agreement with experimental measurements only when one considers that dry and wet samples have straightforwardly different internal friction commonly described by the so-called ()-rheology. The liquid bridges bounding grains not only induce cohesion, but modify the internal friction of the wet assemblies.
The European physical journal. E, Soft matter, Jan 28, 2018
Rheometric measurements on assemblies of wet polystyrene beads, in steady uniform quasistatic she... more Rheometric measurements on assemblies of wet polystyrene beads, in steady uniform quasistatic shear flow, for varying liquid content within the small saturation (pendular) range of isolated liquid bridges, are supplemented with a systematic study by discrete numerical simulations. The numerical results agree quantitatively with the experimental ones provided that the intergranular friction coefficient is set to the value [Formula: see text], identified from the behaviour of the dry material. Shear resistance and solid fraction [Formula: see text] are recorded as functions of the reduced pressure [Formula: see text], which, defined as [Formula: see text], compares stress [Formula: see text], applied in the velocity gradient direction, to the tensile strength [Formula: see text] of the capillary bridges between grains of diameter a, and characterizes cohesion effects. The simplest Mohr-Coulomb relation with [Formula: see text]-independent cohesion c applies as a good approximation for...
Http Www Theses Fr, 2008
Nous reportons ici le comportement rheologique de suspensions concentrees de particules non brown... more Nous reportons ici le comportement rheologique de suspensions concentrees de particules non browniennes de fecule de mais ou de polystyrene spheriques et monodisperses. Le systeme modele de particules de polystyrene est utilise afin de controler les proprietes physicochimiques de la suspension. Ce travail experimental s'articule autour de deux aspects rencontres chez les fluides complexes : le seuil d'ecoulement et le seuil de rheoepaississement. Dans un premier temps, nous avons etudie l'origine du seuil d'ecoulement. Nous avons, a partir de techniques de rheologie classique et locale par IRM, montre que le seuil d'ecoulement des suspensions modeles n'est observe que par l'introduction d'un contraste de densite entre les particules et le fluide suspendant. Ce contraste de densite introduit alors une consolidation a l'echelle des particules. Le seuil d'ecoulement apparait alors sans aucune sedimentation macroscopique de l'echantillon. Dans un second temps, nous avons etudie un phenomene de structuration dynamique, sous ecoulement : le rheoepaississement. Ainsi, en couplant des mesures macroscopiques originales (comme l'etude de l'influence de l'entrefer de la geometrie de mesure sur le comportement rheoepaississant, et des mesures de dilatation volumique en fonction du taux de cisaillement) et des mesures locales des proprietes d'ecoulement par IRM, nous montrons que le rheoepaississement de la suspension de fecule de mais est une consequence directe de la dilatance de Reynolds. Par ailleurs, le systeme modele, contrairement a la suspension de fecule de mais, presente un rheoepaississement qui est accompagne d'un regime de localisation qui devient responsable d'un blocage partiel de l'ecoulement dans l'entrefer de la geometrie de Couette.
We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We com... more We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We combine local velocity and concentration measurements using Magnetic Resonance Imaging with global rheometric experiments. We evidence a continuous transition at very low shear rate from a viscous to a shear thickening, granular, rheology characterized by shear stresses proportional to the shear rate squared. We show that an unexpectedly fast migration of grains occurs towards low shear zones as a result of the granular rheology. Combining local and global measurements allow to show that the interplay between flow and migration is responsible for the macroscopic observation of discontinuous shear thickening, but that locally the shear thickening is continuous.
Journal of Rheology, 2015
Journal of Rheology, 2015
The flow of dry and wet granular media is investigated in a Couette geometry using magnetic reson... more The flow of dry and wet granular media is investigated in a Couette geometry using magnetic resonance imaging in order to test the applicability of the "fluidity model" for nonlocality in these materials. Local volume fraction measurements show that the systems become heterogeneous during flow. We find that the nonlocal rheology of suspensions can be correlated using the fluidity model, but the length scale that emerges is not a material property and the model cannot be used for predictive purposes. Rather, the suspension behavior is fully explained as a consequence of stress-driven particle migration and the resulting concentration gradient. The conclusion is less strong for the dry granular system, but it appears likely that the apparent nonlocal behavior is simply due to the formation of a shear band caused by granular dilatancy. V
Physical review letters, Jan 6, 2015
We study the emergence of discontinuous shear thickening (DST) in cornstarch by combining macrosc... more We study the emergence of discontinuous shear thickening (DST) in cornstarch by combining macroscopic rheometry with local magnetic resonance imaging measurements. We bring evidence that macroscopic DST is observed only when the flow separates into a low-density flowing and a high-density jammed region. In the shear-thickened steady state, the local rheology in the flowing region is not DST but, strikingly, is often shear thinning. Our data thus show that the stress jump measured during DST, in cornstarch, does not capture a secondary, high-viscosity branch of the local steady rheology but results from the existence of a shear jamming limit at volume fractions quite significantly below random close packing.
Rhéoépaississement d'une suspension de fécule de maïs… 4.1 Introduction………………………………………………………………… ... more Rhéoépaississement d'une suspension de fécule de maïs… 4.1 Introduction………………………………………………………………… 4.1.1 Revue bibliographique sur le rhéoépaississant…………………...…. 4.1.2 Rhéologie d'une suspension de fécule de maïs……………………..
Quick clays are unstable soils that can be responsible for severe landslides, like the infamous R... more Quick clays are unstable soils that can be responsible for severe landslides, like the infamous Rissa landslide of april 1978 in Norway. Quick clay landslides are usually attributed to rheology modifications caused by changes in the water content, extraction of the salt by subsurface water drainage, and/or triggering by external mechanical perturbations. However, the extreme instability of these soils has until recently remained poorly understood. Quick clay is a thixotropic material, in which aging competes with shear rejuvenation. We study various samples of a natural quick clay from Tiller, near Trondheim (Norway). Rheometry measurements are done by having the material flow under an imposed constant shear stress. The bifurcation observed in the rheology as a function of the imposed stress magnitude provides a precise measurement of the yield stress, and evidences that it strongly depends on the water content. When the material is resting on a plane slope, the amount of shear stress that it must sustain is related to the plane inclination; if the inclination is large enough, he material below a yield surface parallel to the plane is susceptible to flow. For a sample with a clay content of 61%, the resulting flow is that of a classic yield stress fluid, however for a clay content of 59% a dramatic avalanche regime is observed. In the latter "landslide" regime, flow is strongly localized in a thin lubrication layer on which most of the material slides while undergoing little deformation. This is typically what was observed during the Rissa landslide: houses and trees sliding upright down the slope. The landslide regime also results in a much larger travel distance, and in a final deposit height that is independent of the plane inclination. We propose a simple mechanical model that explains the laboratory landslide observations and allows us to relate them to the mechanical behavior measured in the rheometer. We also prepare a "synthetic quick clay" with mechanical properties identical to those of the natural quick clay at all salt concentrations. We show that the presence of a few percent of swelling clay is crucial, and that there exists a range of salt concentration for which the material's elastic modulus is highly dependent on the salt concentration. This is an evidence of the lansdslide-triggering effect of salt washing by water.
L'Imagerie par Resonance Magnetique, surtout connue comme outil de diagnostic medical, est eg... more L'Imagerie par Resonance Magnetique, surtout connue comme outil de diagnostic medical, est egalement une technique riche d'applications dans le domaine de l'etude des materiaux solides ou liquide. Depuis son installation en 1999, l'IRM du LMSGC a, entre autres, permis de mettre en place au laboratoire un ensemble de techniques innovantes pour la caracterisation des fluides et des milieux granulaires en ecoulement, notamment a travers la conception de dispositifs experimentaux inserables de mise en sollicitation, et l'adaptation et le perfectionnement de methodes de velocimetrie. Nous proposons un tour d'horizon du savoir faire actuel du LCPC en la matiere, illustre par des applications variees : mesure d'ecoulement sur modele reduit, rheometrie assistee par IRM en geometries Couette et Cone-Plan (ciment, boues, grains, suspensions, ...), dynamique d'ecoulement dans une mousse, ... (A). (Voir fiche generale F101003 et fiches specifiques F101004 a F101084).
We study the behavior of dense suspensions of non-Brownian particles. We combine macroscopic rheo... more We study the behavior of dense suspensions of non-Brownian particles. We combine macroscopic rheometric experiments and local velocity and concentration measurements through MRI. In steady state, we find that the material is heterogeneous, while the local rheology presents a continuous transition from a viscous to a shear thickening regime (shear stresses proportional to the shear rate squared, as in dry granular materials) at low shear rate. The heterogeneity is shown to result from unexpectedly fast migration of grains during transient, which is attributed to the emergence of the granular rheology. This migration process is accompanied with macroscopic transient discontinuous shear thickening, which is thus not an intrinsic property of granular suspensions.
Soft Matter, 2012
We study the effect of addition of polyethylene oxide (PEO) on the rheological behavior of suspen... more We study the effect of addition of polyethylene oxide (PEO) on the rheological behavior of suspensions of Laponite. Experiments were performed on mixtures of PEO and Laponite at different concentrations. These mixtures can exhibit very strong shear thickening behavior: under shear, the suspension can become a yield stress material sufficiently strong to support its own weight. Depending on the polymer concentration, we observe continuous or discontinuous shear thickening mechanisms; it is the discontinuous shear thickening that leads to the formation of yield stress materials.
Physics of Fluids, 1998
An analytical study of isotropic scalar fluctuations decay in isotropic turbulence is undertaken.... more An analytical study of isotropic scalar fluctuations decay in isotropic turbulence is undertaken. From a fixed-point analysis, the existence of two complete self-preserving regimes is demonstrated. One of them relates to the final period of decay whereas the other one corresponds to the decay at large Reynolds and Péclet numbers. In both cases, the scalar-to-velocity timescale ratio is constant. In
Physical Review Letters, 2010
We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We com... more We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We combine local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometry experiments. In steady state, we observe that the material is heterogeneous, and we find that that the local rheology presents a continuous transition at low shear rate from a viscous to a shear thickening, Bagnoldian, behavior with shear stresses proportional to the shear rate squared, as predicted by a scaling analysis. We show that the heterogeneity results from an unexpectedly fast migration of grains, which we attribute to the emergence of the Bagnoldian rheology. The migration process is observed to be accompanied by macroscopic transient discontinuous shear thickening, which is consequently not an intrinsic property of granular suspensions.
Physical Review Letters, 2014
We show experimentally that the sliding friction on sand is greatly reduced by the addition of so... more We show experimentally that the sliding friction on sand is greatly reduced by the addition of some-but not too much-water. The formation of capillary water bridges increases the shear modulus of the sand, which facilitates the sliding. Too much water, on the other hand, makes the capillary bridges coalesce, resulting in a decrease of the modulus; in this case, we observe that the friction coefficient increases again. Our results, therefore, show that the friction coefficient is directly related to the shear modulus; this has important repercussions for the transport of granular materials. In addition, the polydispersity of the sand is shown to also have a large effect on the friction coefficient.
Physical Review Letters, 2009
We study the emergence of a yield stress in dense suspensions of non-Brownian particles, by combi... more We study the emergence of a yield stress in dense suspensions of non-Brownian particles, by combining local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometric experiments. We show that the competition between gravity and viscous stresses is at the origin of the development of a yield stress in these systems at relatively low volume fractions. Moreover, it is accompanied by a shear banding phenomenon that is the signature of this competition. However, if the system is carefully density matched, no yield stress is encountered until a volume fraction of 62.7 0.3%.
Physical Review Letters, 2010
Yield stress fluids have proven difficult to characterize, and a reproducible determination of th... more Yield stress fluids have proven difficult to characterize, and a reproducible determination of the yield stress is difficult. We study two types of yield stress fluids (YSF) in a single system: simple and thixotropic ones. This allows us to show that simple YSF are simply a special case of thixotropic ones, and to pinpoint the difference between static and dynamic yield stresses, one of the major problems in the field. The thixotropic systems show a strong time dependence of the viscosity due to the existence of an internal percolated structure that confers the yield stress to the material. Using loaded emulsions to control the thixotropy, we show that the transition to flow at the yield stress is discontinuous for thixotropic materials, and continuous for ideal ones. The discontinuity leads to a critical shear rate below which no steady flows can be observed, accounting for the ubiquitous shear banding observed in these materials.
Physical Review Letters, 2009
We study the rheology of quick clay, an unstable soil responsible for many landslides. We show th... more We study the rheology of quick clay, an unstable soil responsible for many landslides. We show that above a critical stress the material starts flowing abruptly with a very large viscosity decrease caused by the flow. This leads to avalanche behavior that accounts for the instability of quick clay soils. Reproducing landslides on a small scale in the laboratory shows that an additional factor that determines the violence of the slides is the inhomogeneity of the flow. We propose a simple yield stress model capable of reproducing the laboratory landslide data, allowing us to relate landslides to the measured rheology.
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Papers by Abdoulaye Ndao Fall