(450f) Measurements of Structure-Property Relationships in Shear Thickening, Concentrated, near Hard Sphere, Colloidal Dispersions Via 1-2 Plane Flow-SANS
In this work we generate structure-property relationships for shearing, concentrated colloidal dispersions through the use of a novel 1-2 plane flow cell sample environment geometry for SANS measurements (Liberatore et al. PRE, 2006), and compare these to theory. Concentrated particle suspensions show widely varying rheological behavior, including shear thinning and shear thickening, even in the absence of particle interactions other than excluded volume (hard spheres). Colloidal suspensions of hard-spheres begin to show significant shear thinning and shear thickening at volume fractions above about 20%; these suspensions show progressively stronger shear thinning and shear thickening with increasing volume fraction. Shear thickening is known to occur via the formation of load bearing hydroclusters from theory, simulations, flow-SANS experiments, and stress-jump rheological measurements, but there are no previous measurements of the structural rearrangements that accompany shear thinning or thickening directly in the plane of shear (the 1-2 or flow-gradient plane). Here, we study model suspensions of near hard-sphere silica particles, approximately 120nm in diameter in a near contrast-matching Newtonian solvent mixture of deuterated ethylene glycol and polyethylene glycol via rheo-SANS in standard 1-3 shear plane (velocity-vorticity) as well as directly in the 1-2 plane (velocity-gradient). Significant anisotropic structural rearrangements under shear are evident. These microstructure changes are compared to the rheological behavior via stress-SANS laws that separate the thermodynamic and hydrodynamic components of the stress which drive shear thinning and shear thickening, respectively. The results are compared against Stokesian Dynamics simulations and theory to provide validation of the mechanisms of shear thinning nad shear thickening in concentrated colloidal dispersions.