Effect of attractions on shear thickening in dense suspensions

Abstract

Shear thickening in dense suspensions is investigated as the strength of interparticle attraction is increased. Starting with hard spheres, attractions are induced through depletion interactions up to and beyond the point where the suspensions gel and the zero shear rate viscosity diverges. For a range of volume fractions we find that the stress at the onset of shear thickening is a weakly increasing function of the strength of attraction but the extent of shear thickening is diminished. For attractions of sufficient strength to gel the suspension, shear thickening is completely absent. These observations are discussed in terms of a time scale argument for hydrocluster formation and how the shear rate where hydroclusters are formed is influenced by interparticle attractions. For this purpose we develop methods to deconvolute hydrodynamic and thermodynamic contributions to the viscosity and find that the shear rate characterizing the onset of hydrocluster formation is a decreasing function of strength of attraction. Shear thickening is lost because the thermodynamic contributions to the viscosity increase more rapidly than do the hydrodynamic contributions. This conclusion is supported by extension of force balance models for hydrocluster formation to the case where particles experience attractive interactions. These results suggest gelation and shear thickening may be linked as localization phenomena where, in the former case the localization is thermodynamically driven while in the later case, localization is hydrodynamically driven.