Investigation of the solid-liquid transition of highly concentrated suspensions in oscillatory amplitude sweeps

Suspensions of quasimonodisperse polymethylmethacrylate spheres with mean diameters of 4.7 and 3.1 mum, dispersed in a low-molecular-weight polydimethylsiloxane, were characterized using oscillatory shear amplitude sweeps. Thereby, the solid volume concentration was varied. The influence of the sample preparation, the mode of the experiment (controlled shear rate, controlled shear stress), and the parameters of the amplitude sweep such as logarithmic ramp time, measurement time, and frequency were investigated.. The logarithmic ramp and the measurement time were found to be the substantial factors which especially influence the experimental results at low shear stress amplitudes. The frequency has an effect only at higher shear stress amplitudes where the material behaves linearly again. All suspensions showed a Hookean solid behavior at low shear stress amplitudes and a Newtonian fluid behavior at high shear stress amplitudes. In the transition range between the Hookean and the Newtonian behavior the analysis of the primary shear strain-time signal showed that the nonlinear effects were coupled with the occurrence of higher odd harmonics. (C) 2002 The Society of Rheology.