Rheology of oscillating suspensions of noncolloidal spheres at small and large accumulated strains

The oscillatory rheology of a noncolloidal suspension of spheres is studied for volume fractions up to 0.50 and strain amplitudes of oscillation between 0.05 and 5.0. The stress responses to the imposed oscillatory strain can deviate from a linear response, even at the smallest strain amplitude, as quantified by both a Fourier decomposition and a least-squares analysis. The stress component that is in phase with the strain can decay by an order of magnitude over the first 15-20 oscillations, depending upon the concentration and strain amplitude. Over the same number of oscillations, the stress component in phase with the rate of strain remains nearly constant, but does change significantly over a large number of oscillations. Furthermore, the apparent values of the complex viscosity of the suspensions at steady state demonstrate a nonmonotonic dependence on strain amplitude for volume fractions larger than 0.2, confirming previously published data limited to the volume fraction of 0.4. (c) 2011 American Institute of Physics. [doi:10.1063/1.3531745]