Chapter 1 - Non-Newtonian fluid behaviour

Abstract

Fluids can be classified in two different ways, either according to their response to the externally applied pressure or according to the effects produced under the action of a shear stress. The first scheme of classification leads to the so called “compressible” and “incompressible” fluids, depending upon whether the volume of an element of fluid is dependent on its pressure. While compressibility influences the flow characteristics of gases, liquids can normally be regarded as incompressible and what is of greater importance is their response to shearing. This chapter explores the flow characteristics of single phase liquids, solutions, and pseudo-homogeneous mixtures (such as slurries, emulsions, gas-liquid dispersions), which may be treated as a continuum if they are stable in the absence of turbulent eddies, depending upon their response to externally imposed shearing action. A non-Newtonian fluid is one whose flow curve (shear stress versus shear rate) is either non-linear or does not pass through the origin, i.e. where the apparent viscosity, shear stress divided by shear rate, is not constant at a given temperature and pressure, but is dependent on flow conditions such as flow geometry, shear rate, and sometimes even on the kinematics history of the fluid element under consideration. The most common type of time-independent non-Newtonian fluid behavior observed is pseudoplasticity or shear-thinning, characterized by an apparent viscosity that decreases with increasing shear rate.