Normal stresses and free surface deformation in concentrated suspensions of noncolloidal spheres in a viscoelastic fluid

Abstract

Concentrated suspensions of noncolloidal spheres in a constant viscosity elastic fluid were characterized rheologically using rotating plate viscometers and profilometry of the suspension surface deflection near a rotating rod. It was found that the relative viscosity was quantitatively consistent with a previously determined correlation for suspensions based on Newtonian fluids. Moreover, the first normal stress difference N1 was found to be positive and the second normal stress difference N2 negative. Although the magnitude of N1 and N2 increased with the solids volume fraction φ, in general the ratio |N1/N2| decreased as loading increased. Analysis of the normal stress data suggests that the rheological contribution of the solids microstructure was in large part independent of that of the dissolved polymers at high solids loading (φ⩾0.3). The magnitude of N2 at high concentrations approached that measured for similar suspensions in Newtonian fluids, while the magnitude of N1 could be attributed to the viscoelasticity of the suspending fluid. Measurements of the surface deformation of the suspension near a rotating rod at different concentrations and shear rates exhibited three different types of deflection: pure rod climbing, pure rod dipping, and a combination of the two, with an upward climb near the rod and a downward deflection further away from the rod. These observations were found to be qualitatively consistent with the rheological measurements conducted in rotating plate viscometers.