(92e) Shear Thickening of Colloidal Dispersions at High Particle Concentrations- Rheology and SANS Investigations into Particle-Scale Mechanisms

There is substantial interest in the mechanism of shear thickening in concentrated dispersions, with experiments, models, and simulations including various interparticle forces. Shear thickening mechanisms include lubrication hydrodynamics coupled with DLVO potentials, short range nanoscale forces, particle compressibility (elastohydrodynamics), and the possibility of contact friction. The existence of the shear thickened state in colloidal dispersions has been mapped experimentally for a model system of stabilized colloidal silica particles in an organic suspending medium (Cwalina and Wagner, JOR, 2015) that suggests a maximum packing fraction in the shear thickened state of 0.54. Further, direct measurements of the microstructure under flow provide microstructural evidence connecting the measured negative first and second normal stress differences to the predominance of lubrication hydrodynamics in the shear thickened state (Gurnon and Wagner, JFM, 2015). In this work, we explore the properties of the shear thickened state for this same system at volume fractions exceeding this maximum packing fraction by steady and time-dependent rheological measurements of the shear and normal stress differences, as well as complementary neutron scattering measurements of the microstructure under flow and at rest. The transition from negative to positive first normal stress difference is examined with regards to the dilational properties of flows exceeding the maximum packing fraction in the shear thickened state, along with the stability of such flows. These results provide experimental evidence to directly test models and simulations for discontinuous shear thickening in colloidal dispersions.