The rheological dilatancy of concentrated suspensions has been studied to determine the cause of this phenomenon. If monodisperse suspensions of polymeric resins are examined, dilatant viscosity behavior is transformed into a discontinuous viscosity behavior when the volume fraction of solids is raised above 0.50. Experimental evidence is presented which supports the hypothesis that the discontinuity is caused by a flow instability in which surfaces of spheres, packed in a two dimensional hexagonal packing at low shear rates, break up into less ordered arrays of spheres. Although various techniques have been used, white light diffraction from a suspension under shear provides the most dramatic evidence of the ordered packing of the spheres and the order-disorder transition at the instability point.
SynopsisIn 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 can 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, 1974͒ 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.
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