Cage melting and viscosity reduction in dense equilibrium suspensions

Abstract

Zero-shear viscosities of dense colloidal suspensions are strongly correlated with the dynamics of particles in cages produced by nearest neighbors. At a given volume fraction, repulsive or attractive interparticle forces are expected to enhance cages, slow dynamics, and increase zero-shear rate viscosities. However, recent studies have shown that hard sphere glasses melt when depletion attractions are introduced, and this is correlated with the break up of local cages and enhancement of density fluctuation relaxation rates. Glass formation in hard sphere suspensions is attributed to the loss of free volume and the entropic localization of particles. Melting is associated with the ability of depletion attractions to increase the local free volume due to a decrease in nearest-neighbor cages. Here, we report evidence for the reduction in the zero-shear viscosity of dense hard and near-hard sphere suspensions, as the strength of depletion attractions increases, at volume fractions well below the experimental glass and gel transitions. Increasing the strength of depletion attractions is found to drive suspension viscosities through a minimum. The magnitude of the drop in zero-shear viscosity grows in magnitude as the volume fraction is increased. These results suggest that the localization and cage effects that characterize glass formation originate at volume fractions below those associated with glass formation. Experimental results are compared with models that incorporate the effects of microstructure on suspension dynamics.