The effects of interparticle interactions and particle size on reversible shear thickening: Hard-sphere colloidal dispersions

A comparison between the effects of two colloidal stabilizing methods (elec trostatic versus Brownian) on the reversible shear thickening transition in concentrated colloidal suspensions is explored. Five suspensions of monodi sperse silica are synthesized via the Stober synthesis and dispersed in an index matched organic solvent to minimize van der Waals interactions. The r esidual surface charge is neutralized with nitric acid (c(HNO3) approximate to 0.1 M) resulting in a near hard-sphere interaction that is confirmed by small angle neutron scattering measurements across a range of volume fract ions. Rheological measurements demonstrate the effects of neutralization on the low shear and high shear rheology, which show that the onset of shear thickening moves to lower applied shear stresses and scales inversely with particle size cubed, in agreement with theory. Quantitative comparisons of both the low shear viscosity and the critical stress for shear thickening t o predictions for hard spheres and literature data demonstrate the extreme sensitivity of high shear rheology to the surface properties in concentrate d suspensions. (C) 2001 The Society of Rheology.