Shell Thomas Baron Award In Fluid-Particle Systems - Structure, Slip and Rheology of Soft Particle Glasses

Soft particle glasses form a broad family of materials made of deformable particles, as diverse as microgels, emulsion droplets, star polymers, block copolymer micelles and proteins, which are jammed at volume fractions where they are in contact and interact via soft elastic repulsions. Despite a great variability in origin of particle elasticity, soft glasses have many generic features in common. They behave like weak elastic solids at rest but flow very much like liquids above the yield stress. This unique feature is exploited in industry to process high performance coatings, solid inks, ceramics pastes, textured food and personal care products.  These materials also exhibit unusual slip and shear banding depending on whether the surfaces are smooth or rough and the chemical interactions between the particles and surfaces.  Here I will show that the slip and bulk rheology are related to the microstructure, which is controlled by an interplay among chemical, elastic and viscous interactions. 

For bulk rheology, I present a micromechanical 3-d model that quantitatively predicts the non-linear rheology of soft particle glasses. The shear stress and the normal stress differences depend on both the dynamic pair distribution function and the solvent-mediated elastohydrodynamic interactions among the deformed particles. The predictions, which have no adjustable parameters, are successfully validated with experiments on concentrated emulsions and polyelectrolyte microgel pastes, highlighting the universality of the flow properties of soft glasses. These results provide a framework for designing new soft additives with a desired rheological response.

The slip and shear banding is due to variations of the structure near the surfaces, which surprisingly propagates hundreds of particle diameters away from the surface.  This variation and the resulting flow can be described by a non-local fluidity model. Our results establish a link between surface forces, lubrication and yielding in soft glassy or jammed materials and open new routes to manipulate their flow through the surface chemistry of the confining boundaries.