(390e) Rheology of Cubic Particles Suspended in Both a Newtonian Fluid and a Concentrated Colloidal Dispersion

We investigate the flow behavior of concentrated suspensions of cubic zeolite particles (average side length = 3 µm) suspended in a Newtonian fluid (glycerol) under both steady and large amplitude oscillatory shear (LAOS).  Under steady shear, at low shear stresses, the rheology is Newtonian.  Shear thickening is observed beyond a critical value of the shear stress.  The relative viscosity of these suspensions of cubic particles in the shear-thickened state is significantly larger than that for suspensions of spherical particles at comparable volume fractions which can be understood in terms of the lubrication hydrodynamics interactions between spheres and cubes.  In stress-controlled oscillatory flow, Newtonian behavior and dynamic shear thickening are observed but the magnitude of the complex viscosity in the low stress amplitude Newtonian regime is consistently reduced below its steady shear value.  We find the steady shear viscosity of these cubic particles in the Newtonian regime diverges around a volume fraction of 0.67, above random close packing for spheres (0.63) and below that for perfect cubes (0.78).  In contrast, the complex viscosity in the Newtonian regime diverges at a volume fraction of 0.86.  We postulate that this increase in the maximum packing fraction is due to a degree of ordering imparted to the cubic particles by the oscillatory nature of the flow.

            We also explore the rheological consequences of adding the cubic particles to a concentrated colloidal dispersion of near hard-spheres.  The colloidal dispersion exhibits non-Newtonian rheology including shear thinning, a high shear plateau, and shear thickening.  At low volume fractions, the flow curves of these suspensions reflect that of the underlying colloidal dispersion suspending medium.  When plotted against the shear stress, the flow curves can be made to superimpose with a single shift factor.  This simple shifting procedure is violated at higher volume fractions of cubic particles as the shear thinning becomes more pronounced. We show that the increase in the low shear viscosity can be understood through a depletion attraction resulting from overlap of the zeolites’ excluded volume.