The Experts below are selected from a list of 282 Experts worldwide ranked by ideXlab platform
Philippe Coussot - One of the best experts on this subject based on the ideXlab platform.
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Saffman–Taylor Instability in Yield Stress Fluids: Theory–Experiment Comparison
Fluids, 2019Co-Authors: Oumar Abdoulaye Fadoul, Philippe CoussotAbstract:The Saffman–Taylor instability for Yield Stress fluids appears in various situations where two solid surfaces initially separated by such a Material (paint, puree, concrete, yoghurt, glue, etc.) are moved away from each other. The theoretical treatment of this instability predicts fingering with a finite wavelength at vanishing velocity, and deposited Materials behind the front advance, but the validity of this theory has been only partially tested so far. Here, after reviewing the basic results in that field, we propose a new series of experiments in traction to test the ability of this basic theory to predict data. We carried out tests with different initial volumes, distances and Yield Stresses of Materials. It appears that the validity of the proposed instability criterion cannot really be tested under such experimental conditions, but at least we show that it effectively predicts the instability when it is observed. Furthermore, in agreement with the theoretical prediction for the finger size, a master curve is obtained when plotting the finger number as a function of the Yield Stress times the sample volume divided by the square initial thickness, in wide ranges of these parameters. This in particular shows that this traction test could be used for the estimation of the Material Yield Stress.
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Wall slip mechanisms in direct and inverse emulsions
Journal of Rheology, 2018Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Thomas Oerther, Maude Ferrari, Philippe CoussotAbstract:We carry out a series of experiments with the aim of completing our knowledge of wall slip characteristics, through a deductive approach based on macroscopic behavior observations. More precisely, we use model Materials (direct and inverse emulsions) and determine the variations of wall slip properties depending on the Material parameters (droplet size, concentration) and boundary conditions of the flow (free surface or flow between two solid surfaces, normal force, flow beyond Yielding, and coated or rough surface). The wall slip characteristics are determined from long creep tests at different levels and from internal measurements of the velocity profile in the capillary or the Couette flow as determined by magnetic resonance imaging. First, we show that the slip Yield Stress is due either to edge effects in relation with evaporation then pinning around the line of contact or to a kind of adhesion of the suspended elements to the wall. This adhesion effect varies with the characteristics of the solid surface (interaction with elements, roughness), and wall slip (below the Yield Stress) disappears when the adhesion or adherence leads to a wall slip Yield Stress expected to be larger than the Material Yield Stress. Then, we show that, below the Yield Stress, the slip velocity vs shear Stress (from which the slip Yield Stress has been removed) relationship is linear. The corresponding value for the apparent slip layer made of interstitial liquid appears to be independent of the concentration and to vary only slightly with the droplet size. Moreover, it is independent of the normal force (below the critical value inducing elongation) and other experimental conditions, e.g., it is the same for free surface flows. Although the origin of this phenomenon remains to be found, the following scheme appears to be consistent with all observations: the droplets are attracted at a very short distance from the wall, forming regions of the small area in which the liquid layer thickness is very small, the shear Stress being dominated by the shear in these regions. Finally, this apparent layer thickness increases at the approach of the Yield Stress and beyond, or if a slightly rough surface is used, leading to a faster (quadratic?) variation of the slip velocity as a function of the Stress.
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Wall Slip of Soft-Jammed Systems: A Generic Simple Shear Process.
Physical review letters, 2017Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Philippe CoussotAbstract:From well-controlled long creep tests we show that the residual apparent Yield Stress observed with soft-jammed systems along smooth surfaces is an artefact due to edge effects. By removing these effects we can determine the Stress solely associated with steady state wall slip below the Material Yield Stress. This Stress is found to vary linearly with the slip velocity for a wide range of Materials whatever the structure, the interaction types between the elements and with the wall, and the concentration. Thus wall slip results from the laminar flow of some given free liquid volume remaining between the (rough) jammed structure formed by the elements, and the smooth wall. This phenomenon may be described by the simple shear flow in a Newtonian liquid layer of uniform thickness. For various systems this equivalent thickness varies in a narrow range (35 ± 15 nm).
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Solid-solid transition in Landau-Levich flow with soft-jammed systems.
Physical review letters, 2014Co-Authors: M. Maillard, Jalila Boujlel, Philippe CoussotAbstract:We study the Landau-Levich problem, i.e., withdrawal of a plate from a bath of fluid, in the case of a soft-jammed system, which involves a transition from a solid bath to a solid layer stuck on the plate. We show that this solid-solid transition is prepared inside the bath before the emersion from the fluid, through the existence of a uniform (boundary) layer in the liquid regime along the plate. This layer controls the original characteristics of the (solid) coated layer, whose thickness is almost independent of the velocity but proportional to the Material Yield Stress.
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Measuring the surface tension of Yield Stress fluids
Soft Matter, 2013Co-Authors: Jalila Boujlel, Philippe CoussotAbstract:With the aim of studying the impact of capillary forces on the flow of Yield Stress fluids we investigate the properties of a film formed by withdrawing a blade from a bath of such a Material. We show that before a progressive breakage of the film, the force amplitude reaches a maximum which is independent of the initial depth of penetration and the timing for blade lifting, but increases with the Material Yield Stress and the blade thickness. This critical force is shown to reflect both capillary and viscous effects, even at vanishing blade velocity. We demonstrate that the ratio of this force to the blade perimeter provides the surface tension of the Yield Stress fluid in the limit of a low (≪1) capillary number (ratio of Yield Stress times the blade thickness to surface tension). Moreover we show that all our data for the force to perimeter ratio fall along a master curve which may be used to deduce the surface tension from measurements obtained at a capillary number up to 1, even if viscous effects are significant. Finally Carbopol gels appear to have almost the same value of surface tension whatever their Yield Stress, but this value is almost 10% smaller than that of pure water.
Jalila Boujlel - One of the best experts on this subject based on the ideXlab platform.
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Solid-solid transition in Landau-Levich flow with soft-jammed systems.
Physical review letters, 2014Co-Authors: M. Maillard, Jalila Boujlel, Philippe CoussotAbstract:We study the Landau-Levich problem, i.e., withdrawal of a plate from a bath of fluid, in the case of a soft-jammed system, which involves a transition from a solid bath to a solid layer stuck on the plate. We show that this solid-solid transition is prepared inside the bath before the emersion from the fluid, through the existence of a uniform (boundary) layer in the liquid regime along the plate. This layer controls the original characteristics of the (solid) coated layer, whose thickness is almost independent of the velocity but proportional to the Material Yield Stress.
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Measuring the surface tension of Yield Stress fluids
Soft Matter, 2013Co-Authors: Jalila Boujlel, Philippe CoussotAbstract:With the aim of studying the impact of capillary forces on the flow of Yield Stress fluids we investigate the properties of a film formed by withdrawing a blade from a bath of such a Material. We show that before a progressive breakage of the film, the force amplitude reaches a maximum which is independent of the initial depth of penetration and the timing for blade lifting, but increases with the Material Yield Stress and the blade thickness. This critical force is shown to reflect both capillary and viscous effects, even at vanishing blade velocity. We demonstrate that the ratio of this force to the blade perimeter provides the surface tension of the Yield Stress fluid in the limit of a low (≪1) capillary number (ratio of Yield Stress times the blade thickness to surface tension). Moreover we show that all our data for the force to perimeter ratio fall along a master curve which may be used to deduce the surface tension from measurements obtained at a capillary number up to 1, even if viscous effects are significant. Finally Carbopol gels appear to have almost the same value of surface tension whatever their Yield Stress, but this value is almost 10% smaller than that of pure water.
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Measuring the surface tension of Yield Stress fluids
Soft Matter, 2013Co-Authors: Jalila Boujlel, Philippe CoussotAbstract:With the aim of studying the impact of capillary forces on the flow of Yield Stress fluids we investigate the properties of a film formed by withdrawing a blade from a bath of such a Material. We show that before a progressive breakage of the film, the force amplitude reaches a maximum which is independent of the initial depth of penetration and the timing for blade lifting, but increases with the Material Yield Stress and the blade thickness. This critical force is shown to reflect both capillary and viscous effects, even at vanishing blade velocity. We demonstrate that the ratio of this force to the blade perimeter provides the surface tension of the Yield Stress fluid in the limit of a low ("1) capillary number (ratio of Yield Stress times the blade thickness to surface tension). Moreover we show that all our data for the force to perimeter ratio fall along a master curve which may be used to deduce the surface tension from measurements obtained at a capillary number up to 1, even if viscous effects are significant. Finally Carbopol gels appear to have almost the same value of surface tension whatever their Yield Stress, but this value is almost 10% smaller than that of pure water. © 2013 The Royal Society of Chemistry.
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Assessment of penetrometry technique for measuring the Yield Stress of muds and granular pastes
Applied Rheology, 2013Co-Authors: Mayur Tikmani, Jalila Boujlel, Philippe CoussotAbstract:We discuss the possibility of using penetrometry technique for measuring the Yield Stress of concentrations made of grains immersed in a colloidal phase, such as concrete or muds. In that aim we used model Materials made by suspending glass beads at different concentrations in a kaolin-water paste. We then show that a uniform shear Stress develops along the object (plate or cylinder) beyond the entrance length. This shear Stress plotted versus the object velocity exhibits a shape similar to the flow curve of the Material determined from rheometry. For Materials exhibiting the typical flow curve of a simple Yield Stress fluid, i.e. at bead concentrations smaller than 30 %, the Stress associated with an inflection point located at low velocities of this curve appears to correspond to the Material Yield Stress. At larger concentrations of beads the suspensions have a more complex behaviour likely affected by its granular nature at a local scale and the possibility of migration or frictional effects, so that neither conventional rheometry nor penetrometry provide relevant data. We conclude by describing two practical penetrometry techniques for precisely measuring the Yield Stress of simple pastes.
X. Zhang - One of the best experts on this subject based on the ideXlab platform.
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Wall slip mechanisms in direct and inverse emulsions
Journal of Rheology, 2018Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Thomas Oerther, Maude Ferrari, Philippe CoussotAbstract:We carry out a series of experiments with the aim of completing our knowledge of wall slip characteristics, through a deductive approach based on macroscopic behavior observations. More precisely, we use model Materials (direct and inverse emulsions) and determine the variations of wall slip properties depending on the Material parameters (droplet size, concentration) and boundary conditions of the flow (free surface or flow between two solid surfaces, normal force, flow beyond Yielding, and coated or rough surface). The wall slip characteristics are determined from long creep tests at different levels and from internal measurements of the velocity profile in the capillary or the Couette flow as determined by magnetic resonance imaging. First, we show that the slip Yield Stress is due either to edge effects in relation with evaporation then pinning around the line of contact or to a kind of adhesion of the suspended elements to the wall. This adhesion effect varies with the characteristics of the solid surface (interaction with elements, roughness), and wall slip (below the Yield Stress) disappears when the adhesion or adherence leads to a wall slip Yield Stress expected to be larger than the Material Yield Stress. Then, we show that, below the Yield Stress, the slip velocity vs shear Stress (from which the slip Yield Stress has been removed) relationship is linear. The corresponding value for the apparent slip layer made of interstitial liquid appears to be independent of the concentration and to vary only slightly with the droplet size. Moreover, it is independent of the normal force (below the critical value inducing elongation) and other experimental conditions, e.g., it is the same for free surface flows. Although the origin of this phenomenon remains to be found, the following scheme appears to be consistent with all observations: the droplets are attracted at a very short distance from the wall, forming regions of the small area in which the liquid layer thickness is very small, the shear Stress being dominated by the shear in these regions. Finally, this apparent layer thickness increases at the approach of the Yield Stress and beyond, or if a slightly rough surface is used, leading to a faster (quadratic?) variation of the slip velocity as a function of the Stress.
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Wall Slip of Soft-Jammed Systems: A Generic Simple Shear Process.
Physical review letters, 2017Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Philippe CoussotAbstract:From well-controlled long creep tests we show that the residual apparent Yield Stress observed with soft-jammed systems along smooth surfaces is an artefact due to edge effects. By removing these effects we can determine the Stress solely associated with steady state wall slip below the Material Yield Stress. This Stress is found to vary linearly with the slip velocity for a wide range of Materials whatever the structure, the interaction types between the elements and with the wall, and the concentration. Thus wall slip results from the laminar flow of some given free liquid volume remaining between the (rough) jammed structure formed by the elements, and the smooth wall. This phenomenon may be described by the simple shear flow in a Newtonian liquid layer of uniform thickness. For various systems this equivalent thickness varies in a narrow range (35 ± 15 nm).
Julie Goyon - One of the best experts on this subject based on the ideXlab platform.
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Wall slip mechanisms in direct and inverse emulsions
Journal of Rheology, 2018Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Thomas Oerther, Maude Ferrari, Philippe CoussotAbstract:We carry out a series of experiments with the aim of completing our knowledge of wall slip characteristics, through a deductive approach based on macroscopic behavior observations. More precisely, we use model Materials (direct and inverse emulsions) and determine the variations of wall slip properties depending on the Material parameters (droplet size, concentration) and boundary conditions of the flow (free surface or flow between two solid surfaces, normal force, flow beyond Yielding, and coated or rough surface). The wall slip characteristics are determined from long creep tests at different levels and from internal measurements of the velocity profile in the capillary or the Couette flow as determined by magnetic resonance imaging. First, we show that the slip Yield Stress is due either to edge effects in relation with evaporation then pinning around the line of contact or to a kind of adhesion of the suspended elements to the wall. This adhesion effect varies with the characteristics of the solid surface (interaction with elements, roughness), and wall slip (below the Yield Stress) disappears when the adhesion or adherence leads to a wall slip Yield Stress expected to be larger than the Material Yield Stress. Then, we show that, below the Yield Stress, the slip velocity vs shear Stress (from which the slip Yield Stress has been removed) relationship is linear. The corresponding value for the apparent slip layer made of interstitial liquid appears to be independent of the concentration and to vary only slightly with the droplet size. Moreover, it is independent of the normal force (below the critical value inducing elongation) and other experimental conditions, e.g., it is the same for free surface flows. Although the origin of this phenomenon remains to be found, the following scheme appears to be consistent with all observations: the droplets are attracted at a very short distance from the wall, forming regions of the small area in which the liquid layer thickness is very small, the shear Stress being dominated by the shear in these regions. Finally, this apparent layer thickness increases at the approach of the Yield Stress and beyond, or if a slightly rough surface is used, leading to a faster (quadratic?) variation of the slip velocity as a function of the Stress.
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Wall Slip of Soft-Jammed Systems: A Generic Simple Shear Process.
Physical review letters, 2017Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Philippe CoussotAbstract:From well-controlled long creep tests we show that the residual apparent Yield Stress observed with soft-jammed systems along smooth surfaces is an artefact due to edge effects. By removing these effects we can determine the Stress solely associated with steady state wall slip below the Material Yield Stress. This Stress is found to vary linearly with the slip velocity for a wide range of Materials whatever the structure, the interaction types between the elements and with the wall, and the concentration. Thus wall slip results from the laminar flow of some given free liquid volume remaining between the (rough) jammed structure formed by the elements, and the smooth wall. This phenomenon may be described by the simple shear flow in a Newtonian liquid layer of uniform thickness. For various systems this equivalent thickness varies in a narrow range (35 ± 15 nm).
Elise Lorenceau - One of the best experts on this subject based on the ideXlab platform.
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Wall slip mechanisms in direct and inverse emulsions
Journal of Rheology, 2018Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Thomas Oerther, Maude Ferrari, Philippe CoussotAbstract:We carry out a series of experiments with the aim of completing our knowledge of wall slip characteristics, through a deductive approach based on macroscopic behavior observations. More precisely, we use model Materials (direct and inverse emulsions) and determine the variations of wall slip properties depending on the Material parameters (droplet size, concentration) and boundary conditions of the flow (free surface or flow between two solid surfaces, normal force, flow beyond Yielding, and coated or rough surface). The wall slip characteristics are determined from long creep tests at different levels and from internal measurements of the velocity profile in the capillary or the Couette flow as determined by magnetic resonance imaging. First, we show that the slip Yield Stress is due either to edge effects in relation with evaporation then pinning around the line of contact or to a kind of adhesion of the suspended elements to the wall. This adhesion effect varies with the characteristics of the solid surface (interaction with elements, roughness), and wall slip (below the Yield Stress) disappears when the adhesion or adherence leads to a wall slip Yield Stress expected to be larger than the Material Yield Stress. Then, we show that, below the Yield Stress, the slip velocity vs shear Stress (from which the slip Yield Stress has been removed) relationship is linear. The corresponding value for the apparent slip layer made of interstitial liquid appears to be independent of the concentration and to vary only slightly with the droplet size. Moreover, it is independent of the normal force (below the critical value inducing elongation) and other experimental conditions, e.g., it is the same for free surface flows. Although the origin of this phenomenon remains to be found, the following scheme appears to be consistent with all observations: the droplets are attracted at a very short distance from the wall, forming regions of the small area in which the liquid layer thickness is very small, the shear Stress being dominated by the shear in these regions. Finally, this apparent layer thickness increases at the approach of the Yield Stress and beyond, or if a slightly rough surface is used, leading to a faster (quadratic?) variation of the slip velocity as a function of the Stress.
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Wall Slip of Soft-Jammed Systems: A Generic Simple Shear Process.
Physical review letters, 2017Co-Authors: X. Zhang, Elise Lorenceau, Philippe Basset, Tarik Bourouina, Florence Rouyer, Julie Goyon, Philippe CoussotAbstract:From well-controlled long creep tests we show that the residual apparent Yield Stress observed with soft-jammed systems along smooth surfaces is an artefact due to edge effects. By removing these effects we can determine the Stress solely associated with steady state wall slip below the Material Yield Stress. This Stress is found to vary linearly with the slip velocity for a wide range of Materials whatever the structure, the interaction types between the elements and with the wall, and the concentration. Thus wall slip results from the laminar flow of some given free liquid volume remaining between the (rough) jammed structure formed by the elements, and the smooth wall. This phenomenon may be described by the simple shear flow in a Newtonian liquid layer of uniform thickness. For various systems this equivalent thickness varies in a narrow range (35 ± 15 nm).