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28 mai 2018: 1 événement

  • Séminaires Invités

    Lundi 28 mai 2018 14:00-15:30 - Guillaume Ovarlez - LOF (UMR5258 CNRS-Solvay-Université de Bordeaux)

    Concentrated Suspensions : Shear-Induced Inhomogeneities and Shear Thickening

    Résumé : Suspensions of non-Brownian particles in a Newtonian fluid mostly present a linear behavior, with a contribution from both hydrodynamic interactions and interparticle contact forces to their viscosity. An important consequence of the existence of direct contact interactions between the particles is the progressive development of particle concentration inhomogeneities under shear, due to gradients of particle stresses. In addition, close to jamming, some concentrated suspensions also display nonlinear behaviors such as continuous shear thickening (CST) and discontinuous shear thickening (DST). These behaviors are characterized by an increase of the viscosity with the applied shear stress, which is attributed to a transition from lubricated to dry contacts between particles.
    In this talk, I will first discuss how shear-induced inhomogeneities arise in viscous suspensions. I will then focus on the case of concentrated suspensions and will show that macroscopic flows of CST and DST suspensions always display concentration inhomogeneities.
    In the studied CST suspensions, the thickening behavior is associated to the abrupt development of concentration inhomogeneities in the direction of the velocity gradient. This will be shown to be a direct consequence of the increasing role of particle inertia and contact forces in the behavior.
    Two different DST materials are studied and show very different features. In a first suspension, DST is associated to the unexpected development of concentration inhomogeneities in the flow direction, thus observed as a travelling density band. This is observed when particles display only repulsive surface interactions at short distance. This flow/concentration coupling can either be attributed to the steady coexistence of jammed and flowing states, or to an instability of the coupled equations describing the suspension behavior as a function of the fraction of frictional contacts. Both possibilities will be discussed. In a second suspension, the well-known but far from model cornstarch suspension, DST is associated to concentration inhomogeneities in the vorticity direction, also leading to shear banding. With these particles, once a repulsive force is overcome, an adhesion force has to be overcome to separate the particles. A consistent interpretation the observed behaviors is then that : (i) the critical stress of DST is related to the repulsive force that has to be overcome to create contacts ; (ii) due to adhesion, the material has a yield stress fluid behavior above the critical stress, leading to a jump in the apparent viscosity ; (iii) the immersed cohesive granular material thus created is irreversibly compacted under stress, as in cohesive granular materials, leading to zones of higher volume fraction and higher resistance to flow.

    contact : Kirsten Martens

    Lieu : LIPhy, conference room - 140 Avenue de la Physique 38402 Saint Martin d’Hères

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