Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids

Abstract

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...

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