Rheopexy and tunable yield stress of carbon black suspensions

We show that besides simple or thixotropic yield stress fluids there exists a third class of yield stress fluids. This is illustrated through the rheological behavior of a carbon black suspension, which is shown to exhibit a viscosity bifurcation effect around a critical stress along with rheopectic trends, i.e., after a preshear at a given stress the fluid tends to accelerate when it is submitted to a lower stress. Viscosity bifurcation displays here original features: the yield stress and the critical shear rate depend on the previous flow history. The most spectacular property due to these specificities is that the material structure can be adjusted at will through an appropriate flow history. In particular it is possible to tune the material yield stress to arbitrary low values. A simple model assuming that the stress is the sum of one component due to structure deformation and one component due to hydrodynamic interactions predicts all rheological trends observed and appears to well represent quantitatively the data.Comment: submitted to Soft Matte

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

Fluid dynamic treatment of thixotropic debris flows and avalanches

Some forms of mud flows and debris flows exhibit a non-Newtonian thixotropic behaviour, and this study describes a basic study of dam break wave with thixotropic fluid. Theoretical considerations were developed based upon a kinematic wave approximation of the Saint-Venant equations down a prismatic sloping channel and combined with the thixotropic rheological model of Coussot et al. (2002). The analytical solution of basic flow motion and rheology equations predicts three basic flow regimes depending upon the fluid properties and flow conditions, including the initial degree of jamming of the fluid. The present work is the first theoretical analysis combining successfully the basic principles of unsteady flow motion with a thixotropic fluid model, which was verified with systematic laboratory experiments

Drying of a model soil

International audienceDrying experiments have been carried out with model soils made of different pastes filling granular packings. A detailed information concerning the time evolution of the water density distribution inside the sample was obtained from MRI measurements. This study makes it possible to understand the physical origin of the drying characteristics of these materials. The drying curves exhibit a CRP (constant-rate period) and a FRP (falling-rate period) but the relative durations of these periods depend on the paste structure. With a kaolin suspension the CRP lasts down to very low water densities and is associated with a homogeneous drying of the paste throughout the sample. With a bentonite suspension the CRP is shorter and the drying in the FRP results from a complex process involving fractures progressing downwards through the pasty matrix. With a gel the CRP period is even shorter and the drying in the FRP results from the progression of a dry front through the packing as a result of the shrinkage of the gel matrix. This provides an overview of the main possible processes at work when drying a soil as a function of its components, along with some practical means for slowing down drying from soils

Physical origin of shear-banding in jammed systems

Jammed systems all have a yield stress. Among these materials some have been shown to shear-band but it is as yet unclear why some materials develop shear-band and some others do not. In order to rationalize existing data concerning the flow characteristics of jammed systems and in particular understand the physical origin of such a difference we propose a simple approach for describing the steady flow behaviour of yield stress fluids, which retains only basic physical ingredients. Within this frame we show that in the liquid regime the behaviour of jammed systems turns from that of a simple yield stress fluid (exhibiting homogeneous flows) to a shear-banding material when the ratio of a characteristic relaxation time of the system to a restructuring time becomes smaller than 1, thus suggesting a possible physical origin of these trends

Rhéologie des boues de stations d'épuration : études préliminaires pour la maîtrise des stockages et épandages

Les boues résiduaires pâteuses sont des matériaux au comportement complexe. Trop fluides pour tenir sous forme de tas, elles sont néanmoins trop solides pour être épandues correctement avec les moyens mécaniques actuellement utilisés en agriculture. Aussi, afin de réduire ces deux principales entraves à la valorisation agricole, et hormis les différentes réticences d'ordre psychologique, une étude du comportement mécanique de ces boues a été menée récemment. Dans cette optique, et après avoir décrit les filières de traitement des boues, nous nous sommes attachés à mettre en évidence les caractéristiques jugées pertinentes. Des parallèles ont été établis dans ce but et dans plusieurs domaines avec d'autres matériaux, tout en gardant à l'esprit les spécificités de la boue. Enfin, dans le cadre de la mise au point de notre démarche expérimentale, nous avons illustré notre propos par un cas concret. / Soft residual sewage sludge behaves in a complex way. It is too liquid to form into a heap but too solid to be spread properly using current machinery. A study of the mechanical behaviour of this sludge has been undertaken recently to remove these two major obstacles to agricultural recycling, other than various psychological reservations. After a preliminary description of sewerage treatment processes, we have attempted to highlight the characteristics considered to be the most relevant. Parallels are drawn with other materials in several fields, bearing in mind the specific characteristics of sludge. Finally, in order to finalise our experimental approach, we illustrate our proposal with a specific case

On the existence of a simple yield stress fluid behavior

Materials such as foams, concentrated emulsions, dense suspensions or colloidal gels, are yield stress fluids. Their steady flow behavior, characterized by standard rheometric techniques, is usually modeled by a Herschel-Bulkley law. The emergence of techniques that allow the measurement of their local flow properties (velocity and volume fraction fields) has led to observe new complex behaviors. It was shown that many of these materials exhibit shear banding in a homogeneous shear stress field, which cannot be accounted for by the standard steady-state constitutive laws of simple yield stress fluids. In some cases, it was also observed that the velocity fields under various conditions cannot be modeled with a single constitutive law and that nonlocal models are needed to describe the flows. Doubt may then be cast on any macroscopic characterization of such systems, and one may wonder if any material behaves in some conditions as a Herschel-Bulkley material. In this paper, we address the question of the existence of a simple yield stress fluid behavior. We first review experimental results from the literature and we point out the main factors (physical properties, experimental procedure) at the origin of flow inhomogeneities and nonlocal effects. It leads us to propose a well-defined procedure to ensure that steady-state bulk properties of the materials are studied. We use this procedure to investigate yield stress fluid flows with MRI techniques. We focus on nonthixotropic dense suspensions of soft particles (foams, concentrated emulsions, Carbopol gels). We show that, as long as they are studied in a wide (as compared to the size of the material mesoscopic elements) gap geometry, these materials behave as 'simple yield stress fluids': they are homogeneous, they do not exhibit steady-state shear banding, and their steady flow behavior in simple shear can be modeled by a local continuous monotonic constitutive equation which accounts for flows in various conditions and matches the macroscopic response.Comment: Journal of Non-Newtonian Fluid Mechanics (2012) http://dx.doi.org/10.1016/j.jnnfm.2012.06.00

An Experimental Study of Sudden Release of Bentonite Suspensions down an Inclined Chute

Bentonite suspensions, used in the construction industry, are non-Newtonian fluids with a thixotropic behaviour. Sudden releases of bentonite suspensions were systematically investigated down a sloping chute, to quantify the effects of bentonite concentrations and initial rest period on flow motion. Experiments observations highlighted four types of flows, that differ substantially from Newtonian fluid motion. Quantitative informations were documented in terms of the fluid thickness, wave front position and wave front curvature during motion and after stoppage. It is believed that the present study is the first systematic study of its kind in a large-size facility

Modeling the rheological behavior of waxy crude oils as a function of flow and temperature history

International audienceSynopsis The solidification of waxy components during the cool down of waxy crude oils in pipelines may provide complex yield stress fluid behavior with time-dependent characteristics, which has a critical impact for predicting flow restart after pipeline shut-in. Here, from a previous set of data at a local scale with the help of Magnetic Resonance Imaging and a new full set of data for various flow and temperature histories, we give a general picture of the rheological behavior of waxy crude oils. The tests include start flow tests at different velocities or creep tests at different stress levels, abrupt changes of velocity level, steady flow, after cooling under static or flowing conditions. We show that when the fluid has been cooled at rest it forms a structure that irreversibly collapses during the startup flow. Under these conditions, the evolution of the apparent viscosity mainly depends on the deformation undergone by the fluid for low or moderate deformation and starts to significantly depend on the shear rate for larger values. Even the (apparent) flow curve of statically cooled waxy crude oils was observed to be dependent on the flow history, more specifically on the maximum shear rate experienced by the material. After being sufficiently sheared, i.e., achieving an equilibrium state, the rheological behavior is that of a simple liquid for shear rates lower than the maximum historical one. A model is proposed to represent those trends experimentally observed. In contrast with most previous works in that field, the model is built without any a priori assumption based on classical behavior of a class of fluids. Finally, it is shown that this mode

Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids

International audienceWe study experimentally the behavior of isotropic suspensions of noncolloidal particles in yield stress fluids. This problem has been poorly studied in the literature, and only on specific materials. In this paper, we manage to develop procedures and materials that allow us to focus on the purely mechanical contribution of the particles to the yield stress fluid behavior, independently of the physicochemical properties of the materials. This allows us to relate the macroscopic properties of these suspensions to the mechanical properties of the yield stress fluid and the particle volume fraction, and to provide results applicable to any noncolloidal particle in any yield stress fluid. We find that the elastic modulus-concentration relationship follows a Krieger-Dougherty law, and we show that the yield stress-concentration relationship is related to the elastic modulus-concentration relationship through a very simple law, in agreement with results from a micromechanical analysis