Something old, something new, in rheometry
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We consider developments in small amplitude oscillatory-shear testing, from the advent of the first post second world war commercial rheometers to the present day. To facilitate such a survey, we concentrate on the case where the test fluid is contained between two plates, which are initially parallel and horizontal. In the earliest experiments, the bottom plate performed small amplitude torsional oscillations about a vertical axis, with the motion of the upper plate constrained by a torsion wire. Somewhat later, a new kind of rheometer appeared, in which both plates rotated with the same angular velocity about two axes, which were both normal to the plates, but not coincident, (the so-called ‘Orthogonal Rheometer’). This led to an impressive (but relatively short-lived) period of research activity, before the device fell out of favour. At about the same time, a ‘tilted-plate rheometer’ was proposed and the basic theory developed. However, there is no evidence that such a technique was ever investigated experimentally. In recent years, there has been a growing interest in the so called ‘Compressional’ technique, in which the bottom plate is stationary, while the top plate performs small amplitude oscillations in the vertical direction. Such a device is shown to have some advantages over the earlier techniques, especially at high frequencies. Of particular interest in the present study is the way that the effect of ‘fluid inertia’ has been accommodated in the various experimental techniques and some recent theoretical and experimental work carried out at the author’s laboratory on the Compressional instrument is discussed in detail.
Key wordslinear viscoelasticity parallel-plate geometry fluid inertia
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