A new versatile rheological instrument: Design, testing and data analysis

Abstract

Recent work in rheology has indicated the need to explore methods of testing viscoelastic fluids beyond the simple measurement of steady shearing viscosity; we do not imply that there is no longer any need to measure steady shearing viscosity, but there are already many instruments that are capable of making such measurements (1, 2). We present in this paper a short account of the design and proof testing of a new instrument capable of testing fluids and solids in many different modes of deformation derived from rotary motion complementing existing instruments for viscosity measurement (2) and tensile testing (3) which are based on linear motion. If a rotary instrument is used, then the basic type of flow in the sample will naturally be a shearing motion (steady or unsteady) or some deviation from a solid-body rotation. In table 1 we show a classification of shearing motions based on the Pipkin (4) flow diagnosis diagram. Note that the sample response depends on the sample mean relaxation time (T) and on the shearing amplitude (A). These parameters lead to a dimensionless classification in terms of ωT and A where ω is a characteristic rate or frequency describing the rate of change of stress at a particle. In a time-sinusoidal motion ω is the frequency of the input, for example, and in steady shearing ω is zero. The shear amplitude A is easy to define in a sinusoidal motion; in steady shear it may be thought of as the amount of shearing taking place in one relaxation time, so that here we may set A = γT, where γ is the steady shear rate.