Rheological parameters for the viscosity viscoelasticity and thixotropy of blood 1

Abstract

The rheological properties of blood, nonNewtonian viscosity in steady flow, frequency dependence and shear rate dependence of viscoelasticity in oscillatory flow, and thixotropy, are brought together by a unifying concept. Rheological states are defined which separate nonequilibrium properties, such as thixotropy, from equilibrium properties, such as steady flow viscosity and sustained oscillatory flow viscoelasticity. It is considered that the aggregation of erythrocytes is the primary process governing the conditions of equilibrium. A generalized Maxwell model is developed to provide a basis for quantitative analysis of equilibrium properties. A shear rate dependent degradation function serves to adjust the model elements to the flow conditions. Characteristic relaxation times become significant rheological parameters for equilibrium viscosity and viscoelasticity while other characteristic times are important to thixotropy. Numerical data are evaluated for the several rheological properties by comparison with the theory using a computerized regression analysis. These determinations show that nonNewtonian viscosity and viscoelasticity can be calculated using the same numerical properties. Thus, the theory provides a rational framework into which several rheological tests of blood can be placed.