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

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

A Study of Dam Break Wave of Thixotropic Fluid: Bentonite Surges down an Inclined plane

Thixotropic fluids are commonly used in the construction industry (e.g. liquid cements, liquid concrete, drilling fluids), industrial applications (e.g. muds, paints) and the food industry (e.g. liquid dairy products, ketchup). Related applications include some forms of mud flows and debris flows, pasty sewage sludges and some wastewater treatment residues. Thixotropy is the characteristic of a fluid to form a gelled structure over time when it is not subjected to shearing and to liquefy when agitated. A thixotropic fluid is a non-Newtonian fluid with a viscosity that is a function of both shear rate y and instantaneous state(s) of structure of the material. Such a fluid exhibits a reversible time-dependent decrease in apparent viscosity under shear rate and a gradual recovery when the shear stress is removed. This report describes a basic study of dam break wave with thixotropic fluid. A dam break wave is a sudden release of a mass of fluid in a channel. This type of flows has not been studied to date with thixotropic fluid, despite its practical applications : e.g., mudflow release, concrete tests including L-Box and J-Ring for self-consolidating concrete testing, paint applications. Theoretical considerations were developed based upon a kinematic wave approximation of the Saint-Venant equations for a thixotropic fluid down a prismatic sloping channel. The thixotropic fluid model of COUSSOT et al. (2002a) was used since it describes the instantaneous state of fluid structure by a single parameter. The analytical solution of the basic flow motion and rheology equations predict three basic flow regimes depending upon the fluid properties and flow conditions, including the initial degree of jamming of the fluid : (1) a short motion with relatively-rapid flow stoppage for relatively small mass of fluid, (2) a fast flow motion for a large mass of fluid, or (3) an intermediate motion initially rapid before final fluid stoppage for intermediate mass of fluid and intermediate initial rest period To. Physical experiments were performed with bentonite suspensions. Systematic experiments showed four types of flows. For small bentonite mass concentrations and short relaxation times To, the fluid flowed rapidly down the slope and spilled into the overflow container (Flow Type I). For intermediate concentrations and rest periods, the suspension flowed rapidly initially, decelerated relatively suddenly, continued to flow slowly for sometimes before complete stoppage (Flow Type II). For large mass concentrations and long rest periods, the mass of fluid stretched down the slope, until the head separated from the tail (Flow Type III). The last flow pattern (Type IV) corresponded to an absence of flow for large bentonite concentrations and long rest times. Quantitative informations were documented in terms of the final fluid thickness, wave front position, wave front curvature, side profile of the wave front during motion and after stoppage, as well as the flow motion immediately after gate opening. Some freesurface instabilities are also discussed and illustrated. It is believed that the present study is the first theoretical analysis combining successfully the basic principles of unsteady flow motion (i.e. Saint-Venant equations) with a thixotropic fluid model, which was validated with large-size systematic laboratory experiments. It is the belief of the writers that, for such complex systems this kind of approach, combining both rheology and fluid dynamics, is necessary to gain new insights of these complicated flow motions

Yielding and flow of foamed metakaolin pastes

Metakaolin is a broadly used industrial raw material, with applications in the production of ceramics and geopolymers, and the partial replacement of Portland cement. The early stages of the manufacturing of some of these materials require the preparation and processing of a foamed metakaolin-based slurry. In this study, we propose to investigate the rheology of a foamed metakaolin-based fresh paste by performing well-controlled experiments. We work with a non-reactive metakaolin paste containing surfactant, in which we disperse bubbles of known radius at a chosen volume fraction. We perform rheometry measurements to characterize the minimum stress required for the foamed materials to flow (yield stress), and the dissipation occurring during flow. We show that the yield stress of the foamed samples is equal to the one of the metakaolin paste, and that dissipation during flow increases quadratically with the bubble volume fraction. Comparison with yielding and flow of model foamed yield stress fluids allows us to understand these results in terms of coupling between the bubbles' surface tension and the metakaolin paste's rheology

Rhéologie de suspensions de bulles

Un dispositif original a été développé pour fabriquer des suspensions monodisperses de bulles (diamètre 700µm) dans un fluide newtonien très visqueux (viscosité dynamique 100Pa.s). La viscosité de ces suspensions a été mesurée dans les régimes dilué et faiblement concentré (fraction volumique en bulles comprises entre 2% et 15%) dans une géométrie Couette. La viscosité des suspensions dépend de la viscosité du fluide suspendant, de la fraction volumique en bulles et d'un nombre sans dimension (nombre capillaire) caractérisant la déformation des bulles par rapport à celle du fluide suspendant. Les résultats expérimentaux obtenus se comparent très bien aux prédictions du modèle de Frankel et Acrivos [1] dans la gamme des fractions volumiques étudiée

MRI investigation of granular interface rheology using a new cylinder shear apparatus

The rheology of granular materials near an interface is investigated through proton magnetic resonance imaging. A new cylinder shear apparatus has been inserted in the MRI device, which allows the control of the radial confining pressure exerted by the outer wall on the grains and the measurement of the torque on the inner shearing cylinder. A multi-layer velocimetry sequence has been developed for the simultaneous measurement of velocity profiles in different sample zones, while the measurement of the solid fraction profile is based on static imaging of the sample. This study describes the influence of the roughness of the shearing interface and of the transverse confining walls on the granular interface rheology

Macroscopic vs local rheology of yield stress fluids

International audienceFrom MRI velocimetry we measure the local flow characteristics of a Carbopol gel in a Couette geometry under different inner cylinder rotation velocities. Associated with torque data under the same flow conditions we deduce the local, steady-state, simple shear, constitutive equation of the material within a relatively wide range of sheFrom MRI velocimetry we measure the local flow characteristics of a Carbopol gel in a Couette geometry under different inner cylinder rotation velocities. Associated with torque data under the same flow conditions we deduce the local, steady-state, simple shear, constitutive equation of the material within a relatively wide range of shear rates [10−2; 100 s−1]. Then we show that in this range of shear rates this “local” behaviour is in excellent agreement with the “macroscopic” behaviour deduced from conventional rheometry with cone and plate and Couette geometries. We can conclude that this material effectively behaves as a simple yield stress fluid with a constitutive equation well represented by a Herschel–Bulkley model. This behaviour, likely due to the soft-jammed structure of the fluid, contrasts with that of aggregated systems which exhibit thixotropy and shear-banding at low shear rates.ar rates [10−2; 100 s−1]. Then we show that in this range of shear rates this “local” behaviour is in excellent agreement with the “macroscopic” behaviour deduced from conventional rheometry with cone and plate and Couette geometries. We can conclude that this material effectively behaves as a simple yield stress fluid with a constitutive equation well represented by a Herschel–Bulkley model. This behaviour, likely due to the soft-jammed structure of the fluid, contrasts with that of aggregated systems which exhibit thixotropy and shear-banding at low shear rates

Viscous friction of squeezed bubbly liquid layers

International audienceShear viscosity of bubbly liquids is known to depend on both the gas volume fraction and the capillary number. Here we study the impact of confinement on their behavior by investigating the viscosity of semi-dilute bubbly liquid layers confined between two plates and characterized by a ratio of the undeformed bubble diameter to the layer thickness equal or larger than unity. For all the studied confinement ratios viscosity is shown to be smaller than the viscosity of the suspending liquid for capillary numbers larger than 0.1. Measurements of bubble deformations show that this behavior is related to bubble stretching in the direction of shear induced flow. In the limit of high capillary numbers, viscosity reaches values predicted for unconfined bubbly liquids. On the other hand, our results for smaller capillary numbers, i.e. within the range 0.001-0.1, reveal a non-monotonic variation of the viscosity as a function of the confinement ratio, exhibiting a well-defined maximum value for ratio close to 1.8. This behavior differs strongly from the reference case of unconfined bubbly liquid, and it is shown to result from both bulk and wall drag forces for the squeezed bubbles

Steady shear viscosity of semi-dilute bubbly suspensions

International audienceThe steady shear viscosity of bubbly suspensions is known to depend upon the suspending fluid viscosity, the bubble volume fraction and a dimensionless number (the capillary number) accounting for the deformability of the bubbles under a viscous stress. However, experimental data on bubbly suspensions in the literature are scarce and present two main shortcomings: (i) the studied systems are polydisperse, which leads to arbitrariness in the definition of a capillary number, and (ii) there is a lack of data for capillary numbers of order unity, where bubbles are slightly deformable. This leads to the absence of clear conclusion on the relevance of the existing theoretical models. In order to make significant progress, an original device is designed to produce monodisperse suspensions of bubbles in a highly viscous Newtonian fluid. The steady shear viscosity of the bubbly suspensions is measured in the dilute and semi-dilute concentration regimes (volume fraction of bubbles between 2% and 18%) using a Couette geometry with a 20-bubbles-wide gap, for capillary numbers ranging between 0.01 and 10. The new obtained data are shown to be much more accurate than data previously reported in the literature. Experimental data are in very good agreement with the model of Frankel and Acrivos (1970), originally developed for dilute suspensions, in the whole range of studied volume fractions

Yielding and flow of foamed metakaolin pastes

International audienceWe perform an experimental study of the rheology of a foamed non-reactive particulate paste. The paste is a concentrated suspension of metakaolin, a material commonly used in the production of ceramics and in the construction industry. Most manufacturing processes of porous ceramics or foamed building materials require the preparation and handling of a foamed slurry. The slurry is a concentrated particulate paste, which has a non-Newtonian rheology. In particular, a minimum stress is necessary to make it flow: it is a yield stress fluid. We systematically investigate the influence of bubble addition on the workability of our metakaolin slurry by dispersing bubbles of known radius at a chosen volume fraction in the surfactant-laden metakaolin paste. We perform rheometry measurements to characterize the minimum stress required for the foamed materials to flow (yield stress), and the dissipation occurring during flow. We show that the yield stress of the foamed samples is equal to that of the metakaolin paste, and that dissipation during flow increases quadratically with the bubble volume fraction. We show that this behaviour can be well understood from the recent results of Ducloué et al. [7]. This agreement suggests that the results we present here on foamed metakaolin samples may apply to a broader class of foamed particulate pastes