Description of Shear Viscosity Curves with Six Semi-Empirical Model Constants
Petr Filip, Jiri David
Institute of Hydrodynamics, Acad.Sci.Czech Rep.
Czech Republic
Keywords: Viscosity, Model, Non-Monotonicity
To our knowledge, a little attention has been hitherto paid to the systematic attempts how to model semi-empirically the shear stress - shear rate relation of flow behaviour in steady simple shear conditions for a broad class of various materials ranging from aqueous surfactant solutions, bituminous materials, associative polymers, polymer thickeners, lacquers and gels, to some special disperse systems.
On the other hand, a vast number of measured flow curves depicting behaviour of these systems has been reported in the literature. When the viscometric data are presented in terms of viscosity dependence on the logarithm of shear rate (or alternatively on the logarithm of shear stress), some non-standard features in the course of the experimental viscosity function are pronounced with the materials mentioned.
Typically, the non-monotonicity of the viscosity function is present, depending on some decisive parameters of the system - as e.g. temperature, concentration of the surfactant, the chemical nature of thickener, polymer additives to asphalts, concentration of dispersed phase, etc. This non-monotonous course of viscosity exhibits local maximum or minimum, or their sequence along the flow curve. In some cases the regions - with levelling off the viscosity to another slope leading to an inflection point or interval on the viscosity curve - can be observed. Sometimes this region can be classified as an intermediate Newtonian plateau, positioned in-between the lower and upper Newtonian viscosities. The effects described can sometimes occur in combination and with different degree of magnitude and more or less separated one from the other.
Based on the structural approach, some features of the ‘anomalies’ mentioned above have been successfully explained and even quantitatively described e.g. from the macromolecular viewpoint. Despite of this success there are no simple rheological models by means of which the whole extent of the viscosity curve could be expressed using a straightforward means. In other words, it does not exist a relatively simple viscosity model containing sufficiently small number of model parameters, which - moreover - could be easily related to the measured viscosity data. The development of such viscosity model is described in this contribution, together with its application to the materials listed above.