Rheological Properties and Small-Angle Neutron Scattering of a Shear Thickening, Nanoparticle Dispersion at High Shear Rates

The rheological properties and flow microstructure of a stable dispersion of spherical nanoparticles (d = 32 nm) are investigated and compared to the behavior of colloidal dispersions. The shear rheology and flow-small-angle neutron scattering of charge-stabilized silica nanoparticles dispersed in ethylene glycol are reported as a function of shear stress and particle volume fraction. A custom-built high-shear cone is employed to reach shear rates in excess of 10^4 s^-1 in order to study shear thickening. The results are compared to previous model system studies on colloidal dispersions and to micromechanical models that relate the physical parameters of the system to the rheological response. Reversible shear thickening is observed at very high shear rates, and the measured transition stresses for shear thickening compare well to theoretical predictions for colloidal dispersions. Flow-small-angle neutron scattering measurements both in the radial and tangential orientations show a shear-induced structure near the shear thickening transition that is consistent with the hydrocluster mechanism of reversible shear thickening operative in colloidal dispersions. Hence, continuum fluid mechanics, and, in particular, lubrication hydrodynamics, is confirmed to be operative in the nanometer-scale gaps between the nanoparticle surfaces.