A rheometer for oscillatory studies of nonlinear fluids
In order to study their linear viscoelasticity, fluids and solids are frequently subjected to small-amplitude oscillatory shear in rotational rheometers. Steady rotational shear is employed to characterize nonlinear (non-Newtonian) viscosity, while the shear rate is sometimes programmed to generate a “thixotropic loop” or a step-jump in order to follow the effect on viscosity of the sample’s prior shear history. Recently, however, a number of authors (1–4) have turned to oscillatory shear at large amplitudes, where the response frequently shows a harmonic content resulting from the nonlinear nature of the sample. Further development of this technique is hampered both by difficulties in analyzing and interpreting the data and by the limited capabilities of existing rheometers. The purpose of the present work is to remove some of the difficulties in measurement and analysis, by constructing a new rheometer specifically designed for oscillatory studies of nonlinear fluids. The resultant rheometer-computer system has proven to be suitable. also for small-amplitude oscillations and for steady or time-programmed shear measurements.
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