Explanations for the cause of shear thickening in concentrated colloidal suspensions

In contrast to recent publications suggesting that particle cluster formation alone can play an important role in the shear thickening flow behavior of concentrated colloidal suspensions, we believe that there is little if any substantive evidence to prove it. To support this view, we use data from various studies, including data from studies concluding that layered flow is not involved. One reason for the confusion seems to center around the inability of various light-scattering and neutron-scattering techniques to show particle layering before shear thickening when the layers are not well defined. In this regard, one should understand that layered flow san occur without rigorous ordering of particles within the layers, and as the flowing suspension approaches the point of instability, the hydrodynamic forces driving for the instability will jostle the particles within the layers sufficiently to make it even harder to see the layering and any ordering, if it exists, within the layers. Having these views, we argue that the process described by Hoffman (1972, 1974) for shear thickening is still applicable with refinements. The major refinement is the idea that, after the hydrodynamic forces cause the instability which breaks up the layered flow, particle jamming probably involves cluster formation both with and without particle contact. Particle roughness and angularity will facilitate the contact. Finally, we agree with various authors who argue that the best chance of finding shear thickening in concentrated colloidal suspensions without layer formation lies in Brownian hard-sphere suspensions, but the evidence given for it so far is not definitive. (C) 1998 The Society of Rheology.