Nearly viscometric flows in the new rheometers
In this paper, we consider the flow generated in rheometers which make use of a steady flow to determine the linear time-dependent behaviour of elastico-viscous liquids. The Orthogonal (1), Balance (2) and Eccentric-cylinder (3, 4) rheometers are perhaps the best known examples of this class of rheometers at the present time (see fig. 1). The basic idea is that the test fluid is contained between two instrument members, which rotate with the same angular velocity about axes which differ by a small linear displacement or a small angular displacement. The real and imaginary parts of the complex viscosity are then determined by measuring the components of the force or the components of the couple on one of the instrument members. Provided both instrument members rotate with the same angular velocity and the displacement between the axes of rotation is small enough, the fluid is subjected to a small-amplitude oscillatory shear and the results can be interpreted unambiguously in terms of the complex viscosity.
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