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

Concentrated suspensions of noncolloidal spheres in a constant viscosity el astic fluid were characterized theologically using rotating plate viscomete rs and profilometry of the suspension surface deflection near a rotating ro d. It was found that the relative viscosity was quantitatively consistent w ith a previously determined correlation for suspensions based on Newtonian fluids. Moreover, the first normal stress difference NI was found to be pos itive and the second normal stress difference N-2 negative. Although the ma gnitude of N-1 and N-2 increased with the solids volume fraction phi, in ge neral the ratio \N-1/N-2\ decreased as loading increased. Analysis of the n ormal stress data suggests that the rheological contribution of the solids microstructure was in large part independent of that of the dissolved polym ers at high solids loading (phi greater than or equal to 0.3). The magnitud e of N2 at high concentrations approached that measured for similar suspens ions in Newtonian fluids, while the magnitude of NI could be attributed to the viscoelasticity of the suspending fluid. Measurements of the surface de formation of the suspension near a rotating rod at different concentrations and shear rates exhibited three different types of deflection: pure rod cl imbing, pure rod dipping, and a combination of the two, with an upward clim b near the rod and a downward deflection further away from the rod. These o bservations were found to be qualitatively consistent with the rheological measurements conducted in rotating plate viscometers. (C) 2001 The Society of Rheology.