DYNAMIC VISCOSITY AS A FUNCTION OF SHEAR RATE: THE COMPARISON OF ESTABLISHED RHEOLOGICAL MODELS WITH THE NEWLY DERIVED RHEOLOGICAL MODEL FOR THE ESTIMATION OF ZERO- AND INFINITE-SHEAR RATE VISCOSITY OF VEGETABLE OILS

Abstract. Viscosity is one of the most important physical parameters that need proper measurement in terms of its accuracy, and to be fitted by rheological model to enable interpolation for unknown viscosity. It would be an advantage if the model estimation could be extended for viscosity extrapolation with reliability. Some models are concerned with the accuracy of predicting viscosity within the experimental range value, while others able to predict viscosity at extreme conditions, for instance, viscosity at infinite- and zero-shear rate conditions. Cross and Carreau are able to model viscosity at extreme conditions, but its estimation value at very low shear rate region could not be justified if the experimental data does not signify the presence of Newtonian behaviour. In this study, coconut, corn, canola, and soy oils were investigated with a viscometer at different shear rates, 3.9 – 131.6 s–1, and were sheared at specific temperatures, 50 and 90 °C. The experimental data were curve-fitted with well-known rheological models, and then, a new rheological model was proposed as an alternative equation for viscosity estimation at low shear rate region. Infinite-shear rate viscosity from the new model is consistent with the estimation from well-known models. In addition, results showed that vegetable oil has the characteristic of a pseudoplastic, and the experimental data were well fitted by the new proposed model (R2> 0.96)