(102b) Microstructure Measurements of Concentrated, near Hard Sphere Colloidal Dispersions Via 1-2 Plane Flow-SANS
Concentrated particle suspensions show widely varying rheological behavior, including shear thinning and shear thickening, even in the absence of particle interactions other than their 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 indirect rheological measurements, but there are no previous SANS measurements of the structural rearrangements that accompany shear thinning or thickening directly in the plane of shear (the 1-2 or flow-gradient plane). Shear thinning occurs due to particle rearrangements away from the equilibrium state imposed by Brownian motion and has previously been unmeasured via flow-SANS due to the necessity of measuring in the 1-2 plane of shear. 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 the radial direction (1-3 shear plane) and flow-SANS down the gap (in the 1-2 plane). Volume fractions of 52% and 40% are examined in the 1-2 plane. 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. Measurements in the 1-2 plane allow for the first SANS measurements of the thermodynamic stresses associated with shear thinning. In addition, normal stresses can be measured via SANS and compared to experiment for the first time.