Suspensions of particles and bubbles in yield stress fluids

Many dense suspensions involved in industrial processes (concrete casting, drilling muds, foodstuff transport, etc.) and natural phenomena (debris-flows, lava flows, etc.) are yield stress fluids which contain a significant fraction of coarse rigid particles. In addition, some of these materials contain air bubbles: crystal bearing magmas, aerated food emulsions, foamed plaster slurries… When scale separation between the paste microstructure (the colloidal particle size) and the noncolloidal particles or bubbles in suspension is possible, a simplification occurs: these materials can be considered as noncolloidal particles or bubbles embedded in a yield stress fluid (the colloidal paste). From a fundamental point of view, one then deals with suspensions in which particles interact through nonlinear hydrodynamical interactions.

In this talk, we will present recent advances in the characterization and understanding of the behavior of suspensions of rigid and soft inclusions in yield stress fluids.

We will first show how model materials and appropriate procedures are designed to focus on the purely mechanical contribution of the particles/bubbles to the yield stress fluid behavior, independently of the physicochemical properties of the materials. This allows an in depth comparison with micromechanical estimates. We will then focus on the impact of particles on the linear and nonlinear rheological properties (elastic modulus, yield stress, consistency) of the fluid. The variations of these properties with the particle volume fraction are found to be in good agreement with those predicted by a micromechanical approach in which the concentration of shear between the inclusions is taken into account. In the case of suspensions of bubbles, bubbles can behave as rigid or soft particles, dependant on their physical properties and on the interstitial fluid mechanical properties; we will show that the visco-elastic and visco-plastic behaviors of these materials are governed by two different dimensionless capillary numbers, which gives rise to original properties. Application to industrial materials will finally be presented.