Oscillatory shear of suspensions of noncolloidal particles

Abstract

The rheological behavior of noncolloidal suspensions in oscillatory shear flow was studied by measuring the complex viscosity as a function of total strain. The rheology was evaluated in the Couette and parallel-plate geometries for five different suspension systems at a volume fraction of 0.40. As a reference, the steady shear rheology was also evaluated in both geometries. For steady shear in the Couette geometry, suspensions showed a decrease in viscosity as a result of shear-induced particle migration, whereas in the parallel-plate geometry, no change in viscosity was observed over a similar total strain. The oscillatory shear rheology was observed to have a strong dependence on the applied strain amplitude. At each amplitude, the steady value of the complex viscosity was preceded by a drift. The complex viscosity decreased with total strain for high strain amplitudes and increased for low amplitudes. The transition point at which the qualitative behavior changed occurred at an amplitude-to-gap ratio between 0.1 and 0.5 and was independent of the particle size distribution and suspension system. The large strain required to reach a steady value for the complex viscosity at the lower amplitudes suggests that previous measurements may not have been reported with respect to their steady values. The oscillatory shear results were independent of the shear cell geometry, indicating that shear-induced migration was of no consequence and that the observed behavior was instead due to changes in the suspension microstructure.