We review and compare the phenomenological aspects and physical origin of shear-localization and shear-banding in various material types, namely emulsions, suspensions, colloids, granular materials and micellar systems. It appears that shear-banding, which must be distinguished from the simple effect of coexisting static-flowing regions in yield stress fluids, occurs in the form of a progressive evolution of the local viscosity towards two significantly different values in two adjoining regions of the fluids in which the stress takes slightly different values. This suggests that from a global point of view shear-banding in these systems has a common physical origin: two physical phenomena (for example, in colloids, destructuration due to flow and restructuration due to aging) are in competition and, depending on the flow conditions, one of them becomes dominant and makes the system evolve in a specific direction.

We carried out systematic creep tests after different times of rest and over sufficiently long times with pasty materials of various internal structures in a Couette geometry. From an analysis of the data taking into account the inertia of the system and the heterogeneous distribution of stress, we show that: ͑i͒ for a stress below the yield stress these materials remain solid but undergo residual, irreversible deformations over long time which exhibit some trends typical of aging in glassy systems; ͑ii͒ as a result of thixotropy ͑or aging͒ in the solid regime the elastic modulus increases logarithmically with the time of rest; ͑iii͒ in the liquid regime the effective behavior of the material can be well represented by a truncated power-law model; ͑iv͒ a fundamental parameter of the solid-liquid transition is a critical effective shear rate ͑associated with the yield stress͒ below which the material cannot flow steadily.

The effects of viscosity on the mechanical response of a liquid bridge are investigated in the case of small amounts of liquid axially strained between two moving spheres. An experimental setup allows the measurement of capillary and viscous forces exerted on the spheres as a function of the spheres separation distance and the spheres velocity. The experimental results are found to be accurately described over a large range in spheres velocity and liquid viscosity by a simple closed-form expression. In addition, the bridge rupture distance is found to increase like the square root of the separation velocity. Copyright 2000 Academic Press.

International audienceThe behavior of suspensions of rigid particles in a non-Newtonian fluid is studied in the framework of a nonlinear homogenization method. Estimates for the overall properties of the composite material are obtained. In the case of a Herschel-Bulkley suspending fluid, it is shown that the properties of a suspension with overall isotropy can be satisfactory modeled as that of a Herschel-Bulkley fluid with an exponent equal to that of the suspending fluid. Estimates for the yield stress and the consistency at large strain rate levels are proposed. These estimates compare well to both experimental data obtained by Mahaut et al [J. Rheol. 52, 287-313 (2008)] and to experimental data found in the literature

SynopsisWe 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 focusing 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 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. * corresponding author: guillaume.ovarlez@lcpc.fr † Support from the Agence Nationale de la Recherche (ANR) is acknowledged (grant ANR-05-JCJC-0214). I IntroductionDense suspensions arising in industrial processes (concrete casting, drilling muds, foodstuff transport...) and natural phenomena (debris-flows, lava flows...) often involve a broad range of particle sizes. The behavior of these materials reveal many complex features which are far from being understood (for a recent review, see Stickel and Powell (2005)). This complexity originates from the great variety of interactions between the particles (colloidal, hydrodynamic, frictional, collisional...) and of physical properties of the particles (volume fraction, deformability, sensitivity to thermal agitation, shape, buoyancy...) involved in the material behavior.Basically, these materials exhibit a yield stress and have a solid viscoelastic behavior below this yield stress; above the yield stress they behave as liquids, and their flow behavior is usually well fitted to a Herschel-Bulkley law [Larson (1999)] although the exact details of the constitutive law seem more complex at the approach of the transition between the liquid and the solid regimes [Coussot (2005)]. The yielding behavior originates from the colloidal interactions which create a jammed network of interacting particles [Larson (1999);Coussot (2005)]. If the behavior of dense colloidal suspensions, and more generally of yield stress fluids, have received considerable interest and been widely studied, the influence of the large particles on this behavior have been poorly studied. Moreover, the few existing experimental studies have focused on very specific material e.g. particles in a clay dispersion [Coussot (1997), Ancey and Jorrot (2001)], a cement paste [Geiker et al. (2002)], a foam [Cohen-Addad et al. (2007)] or coal slurries [Sengun and Probstein (1989a,b)]. This poses a problem: can we use the results obtained in studies performed with noncolloidal particles in clay dispersions to predict the behavior of a m...

We investigate the origins of salt damage in sandstones for the two most common salts: sodium chloride and sulfate. The results show that the observed difference in damage between the two salts is directly related to the kinetics of crystallization and the interfacial properties of the salt solutions and crystals with respect to the stone. We show that, for sodium sulfate, the existence of hydrated and anhydrous crystals and specifically their dissolution and crystallization kinetics are responsible for the damage. Using magnetic resonance imaging and optical microscopy we show that when water imbibes sodium sulfate contaminated sandstones, followed by drying at room temperature, large damage occurs in regions where pores are fully filled with salts. After partial dissolution, anhydrous sodium sulfate salt present in these regions gives rise to a very rapid growth of the hydrated phase of sulfate in the form of clusters that form on or close to the remaining anhydrous microcrystals. The rapid growth of these clusters generates stresses in excess of the tensile strength of the stone leading to the damage. Sodium chloride only forms anhydrous crystals that consequently do not cause damage in the experiments.

This paper deals with the removal of a small sphere initially attached to a liquid interface. The sphere is small enough (0.3−1 mm) for the capillary force to dominate the interaction and large enough for the line tension effect to be negligible. We have measured simultaneously the force and the geometric parameters of the system as a function of the relative (sphere/interface) separation distance during the detachment process, with a high precision. This procedure allows us to quantify the effect of the contact angle hysteresis during the detachment process with respect to the force−path curve. It is shown that the previous work, which assumes a constant receding contact angle, does not describe our experimental data when the hysteresis effect dominates. By analytical integration of the capillary force experienced by the sphere during the detachment process, the first closed-form analytical expression for the detachment work was obtained. Comparison with our experimental data and with the existing numerical calculations showed good agreement. The effect of contact angle hysteresis on the detachment work is also quantified.

International audienceWe study the rheological behavior of suspensions of noncolloidal spheres in yield stress fluids (concentrated emulsions). These are good model systems for understanding, e.g., the rheology of fresh concrete or debris flows, and more generally, the behavior of particles dispersed in any nonlinear material. We use magnetic resonance imaging techniques to investigate the flows of these yield stress suspensions in a concentric-cylinder Couette geometry. We extend the theoretical approach of Chateau et al. [J. Rheol. 52, 489–506 (2008)], valid for isotropic suspensions, to describe suspensions in simple shear flows, in which an anisotropic spatial distribution of particles is induced by flow. Theory and experiments show that the suspensions can be modeled by a Herschel–Bulkley behavior of same index as their interstitial fluid. We characterize the increase of their consistency and their yield stress with the particle volume fraction / in the 0%–50% range. We observe a good agreement between the experimental variations of the consistency with / and the theoretical prediction. This shows that the average apparent viscosity of the sheared interstitial material is correctly estimated and taken into account. We also observe shear-induced migration with similar properties as in a Newtonian fluid, which we predict theoretically, suggesting that particle normal stresses are proportional to the shear stress. However, the yield stress at flow stoppage increases much less than predicted. We also show that new features emerge in the rheology of the yield stress fluid when adding particles. We predict and observe the emergence of a nonzero normal stress difference at the yielding transition. We observe that the yield stress at flow start can differ from the yield stress at flow stoppage, and depends on flow history. It is likely a signature of a shear-dependent microstructure, due to the nonlinear behavior of the interstitial fluid, which makes these materials different from suspensions in Newtonian media. This is confirmed by direct characterization of shear-rate-dependent pair distribution functions using X-ray microtomography. This last observation explains why the theory predictions for the consistency can be correct while failing to model the yield stress at flow stoppage: a unique microstructure was indeed assumed as a first approximation. More sophisticated theories accounting for a shear-dependent microstructure are thus needed