Macroscopic Discontinuous Shear Thickening vs Local Shear Jamming in Cornstarch

We study the emergence of discontinuous shear-thickening (DST) in cornstarch, by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI) measurements. We bring evidence that macroscopic DST is observed only when the flow separates into a low-density flowing and a high-density jammed region. In the shear-thickened steady state, the local rheology in the flowing region, is not DST but, strikingly, is often shear-thinning. Our data thus show that the stress jump measured during DST, in cornstach, does not capture a secondary, high-viscosity branch of the local steady rheology, but results from the existence of a shear jamming limit at volume fractions quite significantly below random close packing.Comment: To be published in PR

Rupture energy of a pendular liquid bridge

International audienceWe propose a simple expression for the rupture energy of a pendular liquid bridge between two spheres, taking into account capillary and viscous (lubrication) forces. In the case of capillary forces only, the results are in accordance with curve fitting expressions proposed by Simons et al. [2] and Willett et al. [5]. We performed accurate measurements of the force exerted by liquid bridges between two spheres. Experimental results are found to be close to theoretical values. A reasonable agreement is also found in the presence of viscous forces. Finally, for small bridge volumes, the rupture criterion given by Lian et al. [10] is modified, taking into account additional viscous effects

Direct osmotic pressure measurements on partially neutralized poly(acrylic acid) gels

We present a new experimental device to measure the osmotic pressure in gels. The system has been tested on partially neutralized poly(acrylic acid) gels. The variation of the osmotic pressure with polymer concentration, Φ, and ionization degree, α, is linear, provided that α is not too high (I.e. the counterions do not condense on the chains). These results are in good agreement with theoretical models and with compressibility measurements obtained by light scattering experiments. The very charged gels are also studied, and the results are discussed in the frame of existing theories and experiments

Détermination directe par résonnance magnétique nucléaire du comportement thixotrope et à seuil des suspensions

International audienceWe carried out coupled, controlled velocity, magnetic resonance imaging (MRI)-rheometry experiments with colloidal suspensions. For not too high relative velocity of the tools (< 70 rpm), the velocity profiles between coaxial cylinders are comosed of two parts : close to the inner cylinder the fluid is sheared at a rate larger than a critical, finite value (in contrast with the behavior of an ideal yield stress fluid) while the fluid is not sheared at all close to the outer cylinder. Even in the steady state the poisition (critical radius) of the interface between these two regions depends on previous flow history. In particular it decreases with the time of preliminary rest, while the critical shear rate and shear stress along the interface increase because of fluid restructuring in the static region. Using a new MRI procedure the velocity profiles have also been recorded during transient tests. We thus could obsserve the displacement of the critical radius in time after sudden changes of the imosed rotation velocity. In that case the rheological analysis of the velocity profiles show that the effective behavior in the sheared region does not change significantly with velocity, time, or flow history as a first approximation it can be represented by a simple power-law model with constant parameters. This means that the apparent rheological behavior, i.e., as deduced from usual rheometrical tests without taking into account this discontinuit in shear rate, does not represent the effective behavior of the material. Furthermore, the apparent thixotropy of these fluids might be basically distated by the displacement of the interface between the sheard and unsheared regions.Nous avons couplé des expériences de rhéométrie à vitesse imposée et d'imagerie par résonnance magnétique sur des suspensions colloïdales. Pour des vitesses de rotation pas trop élevées(< 70 tr/min), les profils de vitesse entre les cylindres coaxiaux sont composés de deux parties : 1- à proximité du cylindre intérieur, le fluide est cisaillé à une vitesse supérieure à une valeur critique finie (en opposition avec le comportement d'un fluide à seuil idéal). 2- à proximité dy cylindre extérieur, le flluide n'est absolument pas cisaillé. Même en régime permanent, la position de l'interface entre ces deux zones dépend de l'histoire du matériau. En particulier, la valeur du rayon critique diminue avec le temps de repos préliminaire à toute mesure tandis que les valeurs critiques de vitesse et de contrainte de cisaillement le long de l'interface augmentent parce que le matériau se restructure dans la région non cisaillée. A l'aide d'une nouvelle procédure d'imagerie, les profils de vitesse sont aussi été enregistrés pendant les tests transitoires. On peut alors observer le déplacement du rayon critique en fonction du temps après un brutal changement de vitesse imposé. Dans ce cas, l'analyse rhéologique des profils de vitesse montre que le comportement effectif dans la zone cisaillée ne change pas significativement avec la vitesse, le temps ou la thixotropie : en première approximation, il peut être représenté par un simple modèle en loi-puissance avec des paramètres constants. Ceci montre que l'apparent comportement rhéologique, déduit des expériences de rhéométrie classique, n'est pas représentatif du réel comportament du matériau. En outre, l'apparente thixotropie de ces matériaux pourrait être simplement dictée par le déplacement de l'interface entre les zones cisaillées et non-cisaillées

Coexistence of Liquid and Solid Phases in Flowing Soft-Glassy Materials

International audienc