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%.
Granular materials do not flow homogeneously like fluids when submitted to external stress,but of... more Granular materials do not flow homogeneously like fluids when submitted to external stress,but often form rigid regions that are separated by narrow shear bands where the material yields and flows. This shear localization impacts their apparent rheology, which makes it difficult to infer a constitutive behaviour from conventional rheometric measurements. Moreover, they present a dilatant behaviour, which makes their study in classical fixedvolume geometries difficult. These features led to perform extensive studies with inclined plane flows, which were of crucial importance for the development and the validation of the μ(I) rheology. Our aim is to develop a method to characterize granular materials with rheometrical tools. Using unusual rheometry measurements in an annular shear cell adapted from Boyer et al. (2011), dense granular flows are studied. A focus is placed on the comparison between the present results and the μ(I)-rheology.
Rheometric measurements on assemblies of wet polystyrene bead assemblies, in steady uniform quasi... more Rheometric measurements on assemblies of wet polystyrene bead assemblies, 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. Numerical results and experimental ones agree quantitatively is the intergranular friction coefficient is set to 0.09, suitable for the dry material. Shear resistance and solid fraction are recorded as functions of the reduced pressure p, comparing normal stress to capillary bridge tensile strength. The Mohr-Coulomb relation with p-independent cohesion c applies for p above 2. The assumption that contact force contributions to stress act as effective stresses predicts shear strength quite well throughout the numerically investigated range of parameters.. A generalized Mohr-Coulomb cohesion c is defined, which relates to the dry material internal friction, coordination numbers and capillary force ...
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.
We study the rheology of cornstarch suspensions, a dense system of non-Brownian particles that ex... more We study the rheology of cornstarch suspensions, a dense system of non-Brownian particles that exhibits shear thickening, i.e. a viscosity that increases with increasing shear rate. Using MRI velocimetry we show that the suspension has a yield stress. From classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid and then solid again when it shear thickens. The onset shear rate for thickening is found to depend on the measurement geometry: the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of the Reynolds dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.
Granular materials do not always flow homogeneously like fluids when submitted to external stress... more Granular materials do not always flow homogeneously like fluids when submitted to external stress, but often form rigid regions that are separated by narrow shear bands where the material yields and flows. This shear localization impacts their apparent rheology, which makes it difficult to infer a constitutive behavior from conventional rheometric measurements. Moreover, they present a dilatant behavior, which makes their study in classical fixed-volume geometries difficult. These features led numerous groups to perform extensive studies with inclined plane flows, which were of crucial importance for the development and the validation of the P(I)-rheology. Our aim is to develop a method to characterize granular materials with rheometrical tools. Using rheometry measurements in an annular shear cell, dense granular flows of 0.5mm spherical and monodisperse beads are studied. A focus is placed on the comparison between the present results and the P(I)-rheology.
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 F1010...
must not spill onto p.2 ar X iv :2 10 6. 12 25 8v 1 [ co nd -m at .s of t] 2 3 Ju n 20 21 2 S. De... more must not spill onto p.2 ar X iv :2 10 6. 12 25 8v 1 [ co nd -m at .s of t] 2 3 Ju n 20 21 2 S. Deboeuf and A. Fall the addition of a small amount of liquid in a granular medium creates cohesion properties due to the surface tension of the wetting liquid, which forms pendular bridges attracting grains to their close neighbours. Such a mixture of grains, liquid and air may have a strong solid-like behaviour (Strauch & Herminghaus 2012) that enables the building of sand castles as opposed to dry sand which cannot stabilize under gravity with slopes steeper than the angle of repose (Fiscina et al. 2012; Bocquet et al. 1998). Due to the very high solid fraction of grains in such materials, direct inter-grains contacts play an important part in the physical mechanisms ruling their rheological behaviour, which is strongly influenced by the shape, surface and material properties of the grains (roughness, friction, elasticity, ...). To those micromechanical features, one should add the physi...
Cohesive granular materials may exhibit a much wider variety of microstructures than cohesionless... more Cohesive granular materials may exhibit a much wider variety of microstructures than cohesionless ones, as adhesive forces may stabilize loose particle arrangements spanned by tenuous contact networks. We use DEM-type numerical simulations to explore two important aspects of the mechanical behaviour of these materials: isotropic or oedometric compression, in which the growing applied stress gradually overcomes the tensile strengths in contacts and causes the irreversible collapse of initially loose networks; and the critical states and inertial flows in steady, homogeneous shear, under controlled normal stress. A simple model of capillary cohesion for wet beads in the pendular regime lends itself to quantitative comparisons with laboratory experiments in both cases. A reduced pressure P* is defined, comparing the applied stress to the tensile strength of contacts, and shown to control the transition between loose, tenuous systems akin to colloidal aggregates and much denser cohesion...
Magnetic Resonance Imaging (MRI) is a powerful and non-invasive technique that can be used to rev... more Magnetic Resonance Imaging (MRI) is a powerful and non-invasive technique that can be used to reveal useful information about different types of materials. During an MRI experiment a magnetic resonance signal is induced (according to Faraday’s law of induction) in a device called “probe”. MRI probes are simply near field antennas designed in a specific way in order to produce a homogeneous magnetic field at a specific frequency in the region of interest. MRI is a strong technique to study unsaturated granular materials. However, it suffers from a significant drawback, that is inherent small signal-to-noise ratio. To overcome this problem the probe used for a specific MRI experiment must be optimised. In this work a bird-cage probe operating at 21.3 MHz, optimised to study unsaturated granular materials under shear stress as well as some experimental results will be presented.
Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It ... more Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It is observed in many concentrated suspensions in nature and industry: water or oil saturated sediments, crystal-bearing magma, fresh concrete, silica suspensions, and cornstarch mixtures. Here, we reveal how shear-thickening suspensions flow, shedding light onto as yet non-understood complex dynamics reported in the literature. When shear thickening is important, we show the existence of density fluctuations that appear as periodic waves moving in the direction of flow and breaking azimuthal symmetry. They come with strong normal stress fluctuations of the same periodicity. The flow includes small areas of normal stresses of the order of tens of kilopascals and areas of normal stresses of the order of hundreds of pascals. These stress inhomogeneities could play an important role in the damage caused by thickening fluids in the industry.
Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It ... more Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It is observed in many concentrated suspensions in nature and industry: water or oil saturated sediments, crystal-bearing magma, fresh concrete, silica suspensions, and cornstarch mixtures. Here, we reveal how shear-thickening suspensions flow, shedding light onto as yet non-understood complex dynamics reported in the literature. When shear thickening is important, we show the existence of density fluctuations that appear as periodic waves moving in the direction of flow and breaking azimuthal symmetry. They come with strong normal stress fluctuations of the same periodicity. The flow includes small areas of normal stresses of the order of tens of kilopascals and areas of normal stresses of the order of hundreds of pascals. These stress inhomogeneities could play an important role in the damage caused by thickening fluids in the industry.
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...
We study the emergence of discontinuous shear-thickening (DST) in cornstarch, the well know syste... more We study the emergence of discontinuous shear-thickening (DST) in cornstarch, the well know system for this phenomenon, by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI) measurements. We bring evidence that macroscopic DST is characterized in wide gap with a shear localization, part of the material close to the inner cylinder is flowing and the rest is not. The flow seperates into a low-density flowing and a high-density jammed region. Moreover, the local rheology of the flowing region does not directly reflect DST but, strikingly, is most often shear-thinning. Our data are not consistent with recent theoretical suggestions based on the presumed existence of s-shaped flow curves. Instead, they support that DST should be attributed to the existence of a shear jamming limit at volume fractions quite significantly below random close packing.
The behaviour of wet granular media in shear flow is characterized by the dependence of apparent ... more The behaviour of wet granular media in shear flow is characterized by the dependence of apparent friction μ * and solid fraction Φ S on the reduced pressure P * and the inertia number I. Reduced pressure, P * = σ 22 a 2 /F 0 , compares the applied normal stress σ 22 on grains of diameter a to the tensile strength of contact F 0 (proportional to the surface tension Γ of the liquid and the beads diameter). A specifically modified rotational rheometer is used to characterize the response of model wet granular material to applied shear ratė γ under controlled normal stress σ 22. Discrete Element Method (DEM) simulations in 3D are carried out in parallel and numerical results are compared with experimental ones. Cohesive, inertia, saturation and viscous effects on macroscopic coefficient of friction μ * and solid fraction Φ S are discussed.
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.
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.
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%.
Granular materials do not flow homogeneously like fluids when submitted to external stress,but of... more Granular materials do not flow homogeneously like fluids when submitted to external stress,but often form rigid regions that are separated by narrow shear bands where the material yields and flows. This shear localization impacts their apparent rheology, which makes it difficult to infer a constitutive behaviour from conventional rheometric measurements. Moreover, they present a dilatant behaviour, which makes their study in classical fixedvolume geometries difficult. These features led to perform extensive studies with inclined plane flows, which were of crucial importance for the development and the validation of the μ(I) rheology. Our aim is to develop a method to characterize granular materials with rheometrical tools. Using unusual rheometry measurements in an annular shear cell adapted from Boyer et al. (2011), dense granular flows are studied. A focus is placed on the comparison between the present results and the μ(I)-rheology.
Rheometric measurements on assemblies of wet polystyrene bead assemblies, in steady uniform quasi... more Rheometric measurements on assemblies of wet polystyrene bead assemblies, 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. Numerical results and experimental ones agree quantitatively is the intergranular friction coefficient is set to 0.09, suitable for the dry material. Shear resistance and solid fraction are recorded as functions of the reduced pressure p, comparing normal stress to capillary bridge tensile strength. The Mohr-Coulomb relation with p-independent cohesion c applies for p above 2. The assumption that contact force contributions to stress act as effective stresses predicts shear strength quite well throughout the numerically investigated range of parameters.. A generalized Mohr-Coulomb cohesion c is defined, which relates to the dry material internal friction, coordination numbers and capillary force ...
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.
We study the rheology of cornstarch suspensions, a dense system of non-Brownian particles that ex... more We study the rheology of cornstarch suspensions, a dense system of non-Brownian particles that exhibits shear thickening, i.e. a viscosity that increases with increasing shear rate. Using MRI velocimetry we show that the suspension has a yield stress. From classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid and then solid again when it shear thickens. The onset shear rate for thickening is found to depend on the measurement geometry: the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of the Reynolds dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.
Granular materials do not always flow homogeneously like fluids when submitted to external stress... more Granular materials do not always flow homogeneously like fluids when submitted to external stress, but often form rigid regions that are separated by narrow shear bands where the material yields and flows. This shear localization impacts their apparent rheology, which makes it difficult to infer a constitutive behavior from conventional rheometric measurements. Moreover, they present a dilatant behavior, which makes their study in classical fixed-volume geometries difficult. These features led numerous groups to perform extensive studies with inclined plane flows, which were of crucial importance for the development and the validation of the P(I)-rheology. Our aim is to develop a method to characterize granular materials with rheometrical tools. Using rheometry measurements in an annular shear cell, dense granular flows of 0.5mm spherical and monodisperse beads are studied. A focus is placed on the comparison between the present results and the P(I)-rheology.
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 F1010...
must not spill onto p.2 ar X iv :2 10 6. 12 25 8v 1 [ co nd -m at .s of t] 2 3 Ju n 20 21 2 S. De... more must not spill onto p.2 ar X iv :2 10 6. 12 25 8v 1 [ co nd -m at .s of t] 2 3 Ju n 20 21 2 S. Deboeuf and A. Fall the addition of a small amount of liquid in a granular medium creates cohesion properties due to the surface tension of the wetting liquid, which forms pendular bridges attracting grains to their close neighbours. Such a mixture of grains, liquid and air may have a strong solid-like behaviour (Strauch & Herminghaus 2012) that enables the building of sand castles as opposed to dry sand which cannot stabilize under gravity with slopes steeper than the angle of repose (Fiscina et al. 2012; Bocquet et al. 1998). Due to the very high solid fraction of grains in such materials, direct inter-grains contacts play an important part in the physical mechanisms ruling their rheological behaviour, which is strongly influenced by the shape, surface and material properties of the grains (roughness, friction, elasticity, ...). To those micromechanical features, one should add the physi...
Cohesive granular materials may exhibit a much wider variety of microstructures than cohesionless... more Cohesive granular materials may exhibit a much wider variety of microstructures than cohesionless ones, as adhesive forces may stabilize loose particle arrangements spanned by tenuous contact networks. We use DEM-type numerical simulations to explore two important aspects of the mechanical behaviour of these materials: isotropic or oedometric compression, in which the growing applied stress gradually overcomes the tensile strengths in contacts and causes the irreversible collapse of initially loose networks; and the critical states and inertial flows in steady, homogeneous shear, under controlled normal stress. A simple model of capillary cohesion for wet beads in the pendular regime lends itself to quantitative comparisons with laboratory experiments in both cases. A reduced pressure P* is defined, comparing the applied stress to the tensile strength of contacts, and shown to control the transition between loose, tenuous systems akin to colloidal aggregates and much denser cohesion...
Magnetic Resonance Imaging (MRI) is a powerful and non-invasive technique that can be used to rev... more Magnetic Resonance Imaging (MRI) is a powerful and non-invasive technique that can be used to reveal useful information about different types of materials. During an MRI experiment a magnetic resonance signal is induced (according to Faraday’s law of induction) in a device called “probe”. MRI probes are simply near field antennas designed in a specific way in order to produce a homogeneous magnetic field at a specific frequency in the region of interest. MRI is a strong technique to study unsaturated granular materials. However, it suffers from a significant drawback, that is inherent small signal-to-noise ratio. To overcome this problem the probe used for a specific MRI experiment must be optimised. In this work a bird-cage probe operating at 21.3 MHz, optimised to study unsaturated granular materials under shear stress as well as some experimental results will be presented.
Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It ... more Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It is observed in many concentrated suspensions in nature and industry: water or oil saturated sediments, crystal-bearing magma, fresh concrete, silica suspensions, and cornstarch mixtures. Here, we reveal how shear-thickening suspensions flow, shedding light onto as yet non-understood complex dynamics reported in the literature. When shear thickening is important, we show the existence of density fluctuations that appear as periodic waves moving in the direction of flow and breaking azimuthal symmetry. They come with strong normal stress fluctuations of the same periodicity. The flow includes small areas of normal stresses of the order of tens of kilopascals and areas of normal stresses of the order of hundreds of pascals. These stress inhomogeneities could play an important role in the damage caused by thickening fluids in the industry.
Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It ... more Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It is observed in many concentrated suspensions in nature and industry: water or oil saturated sediments, crystal-bearing magma, fresh concrete, silica suspensions, and cornstarch mixtures. Here, we reveal how shear-thickening suspensions flow, shedding light onto as yet non-understood complex dynamics reported in the literature. When shear thickening is important, we show the existence of density fluctuations that appear as periodic waves moving in the direction of flow and breaking azimuthal symmetry. They come with strong normal stress fluctuations of the same periodicity. The flow includes small areas of normal stresses of the order of tens of kilopascals and areas of normal stresses of the order of hundreds of pascals. These stress inhomogeneities could play an important role in the damage caused by thickening fluids in the industry.
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...
We study the emergence of discontinuous shear-thickening (DST) in cornstarch, the well know syste... more We study the emergence of discontinuous shear-thickening (DST) in cornstarch, the well know system for this phenomenon, by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI) measurements. We bring evidence that macroscopic DST is characterized in wide gap with a shear localization, part of the material close to the inner cylinder is flowing and the rest is not. The flow seperates into a low-density flowing and a high-density jammed region. Moreover, the local rheology of the flowing region does not directly reflect DST but, strikingly, is most often shear-thinning. Our data are not consistent with recent theoretical suggestions based on the presumed existence of s-shaped flow curves. Instead, they support that DST should be attributed to the existence of a shear jamming limit at volume fractions quite significantly below random close packing.
The behaviour of wet granular media in shear flow is characterized by the dependence of apparent ... more The behaviour of wet granular media in shear flow is characterized by the dependence of apparent friction μ * and solid fraction Φ S on the reduced pressure P * and the inertia number I. Reduced pressure, P * = σ 22 a 2 /F 0 , compares the applied normal stress σ 22 on grains of diameter a to the tensile strength of contact F 0 (proportional to the surface tension Γ of the liquid and the beads diameter). A specifically modified rotational rheometer is used to characterize the response of model wet granular material to applied shear ratė γ under controlled normal stress σ 22. Discrete Element Method (DEM) simulations in 3D are carried out in parallel and numerical results are compared with experimental ones. Cohesive, inertia, saturation and viscous effects on macroscopic coefficient of friction μ * and solid fraction Φ S are discussed.
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.
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.
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Papers by Abdoulaye Fall