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Normal stresses in complex fluids lead to new flow phenomena because they can be comparable to, or even larger than, the shear stress. In addition, they are of paramount importance for formulating and testing constitutive equations for predicting nonviscometric flow behavior. Very little attention has thus far been paid to the normal stresses of yield stress fluids, which are difficult to measure. We report the first systematic study of the first and second normal stress differences in both continuous and slow oscillatory shear of three model nonthixotropic yield stress fluids, with N1 > 0 and N2

International audience; Numerous materials, from biopolymers to filled rubbers, exhibit strain softening at high strain amplitudes during a strain sweep in oscillatory rheology: The modulus decreases with increasing deformation. On the other hand, if the nonlinear elastic response is analyzed within a single oscillation cycle (described by a Lissajous curve), these systems are often reported to exhibit strain hardening. We compare strain sweeps and single cycle LAOS (large amplitude oscillatory shear) analyses of stress vs strain on three very different materials. We conclude that the reported strain hardening is due to the use of a tangent modulus in the LAOS analysis, and that the overall rheology remains strain softening. To show that this conclusion is robust, we demonstrate a rescaling of the modulus that collapses the data from all the oscillatory measurements onto a single master curve that clearly exhibits the correct strain softening behavior.

International audience; We study the local and global rheology of non-Brownian suspensions in a solvent that is not density-matched, leading to either creaming or sedimentation of the particles. Both local and global measurements show that the incomplete density matching leads to the appearance of a critical shear rate above which the suspension is homogenized by the flow, and below which sedimentation or creaming happens. We show that the value of the critical shear rate and its dependence on the experimental parameters are governed by a competition between the viscous and the gravitational forces, and present a simple scaling model that agrees with the experimental results from different types of experiments (local and global) in different setups and systems.

We carried out systematic rheometrical tests under controlled stress with smooth and rough parallel disks, along with magnetic resonance imaging (MRI) tests in coaxial cylinder geometry, with foam and a model concentrated emulsion. At low shear stress wall slip appears to occur but the bulk fluid remains static, as proved by the fact that in this regime the apparent shear rate obtained for a given shear stress is inversely proportional to the gap between the disks. At high shear stress data with different surface types and gaps coincide, suggesting that wall slip is negligible in this regime. In parallel, MRI results show that, in contrast with the apparent, simple, yielding behavior observed in usual rheometry, there is an abrupt transition from a finite shear rate to a static one at critical stress. This critical shear rate precisely corresponds to the transition between the two regimes of slip. This suggests that different flow regimes occur with these materials: (1) at low stress, with smooth surfaces a layer of a different material is sheared along the solid surfaces whereas the rest of the fluid does not flow; with rough surfaces there is no flow; (2) beyond a critical stress, for both surface types, the bulk fluid starts to flow but the shear is localized in a thin layer; then the thickness of this layer increases when stress is applied; (3) for both surface types homogeneous flow is obtained only beyond slightly larger stress, which is associated to a critical, apparent shear rate., Des expériences de rhéometrie avec des surfaces lisses et rugueuses ont été réalisées en géométrie plan-plan ainsi que des expériences d'IRM en géométrie à cylindres coaxiaux, sur une mousse et une émulsion concentrée modèle. Aux faibles contraintes un glissement apparaît à proximité du mobile qui induit le cisaillement alors que le reste du matériau demeure immobile. Aux fortes contraintes les données coïncident quelle soit la nature de la surface, ceci nous indiquant que le glissement est négligeable dans ce régime. Un gradient de cisaillement critique correspond précisément à la transition entre ces deux régimes d'écoulement pour les différents types de surface.

We investigate the nonlinear rheological behavior of colloidal suspensions of Laponite, a synthetic clay, driven by a steady and homogeneous shear strain. We show that the external drive leads to a drastic slowing down of the aging dynamics or even, in some cases, in the rejuvenation of the system. Under shear, after a surprisingly long time, the spontaneous aging process observed at rest is suppressed. The system then reaches a non-equilibrium stationary state, characterised by a complex viscosity depending on the applied shear rate. In addition, the glass exhibits a non-Newtonian shear-thinning behavior. These rheological behaviors confirm recent numerical and theoretical predictions., Comment: 17 pages, Latex, 6 figures, figures included in the body of the text, to appear in Journal of Rheology

Most concentrated colloidal suspensions such as cement, drilling fluids, paints, muds, etc., have been considered until now thixotropic fluids with a flow curve of an ideal yield stress fluid. We start by showing from inclined plane tests, intended to determine the yield stress, that these systems in fact exhibit peculiar properties. Unlike ideal yield stress fluids, they stop flowing abruptly below a critical stress, and start flowing at a high velocity beyond a critical stress, which in addition increases with the time of preliminary rest. In order to clarify these features we carried out a complete set of rheometrical tests with a model fluid, a bentonite suspension. Our results show that under controlled stress, in some cases after significant flow, there is bifurcation of the behavior towards either stoppage or rapid shear, depending on the relative values of the imposed and critical stresses. As an immediate consequence, we find that no (homogeneous) steady state flows at a shear rate below a critical value can be obtained. These results can be qualitatively predicted by a simple theoretical model that assumes that the viscosity of the material results from the competition between aging and shear rejuvenation, associated to, respectively, the organization or disorganization of the network of particle interactions. This shows that the flow curve in the steady state of concentrated colloidal suspensions and, more generally, of structured fluids, is strongly affected by their thixotropy.