Non Newtonian Fluids

Introduction

1.1 Classification of fluids

Fluids are defined as the substances which tend to flow by the application of shear force, fluids are characterized by using a property called viscosity. Fluids which follow the Newton’s law of viscosity are known as Newtonian fluids. Fluids which do not follow the Newton’s law of viscosity are known as non-Newtonian fluids.

A non-Newtonian fluid is defined as the one whose flow curveshowing the shear stress versus shear rate is 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, etc. and sometimes even on the

1.2 Stress strain relation for time-dependent fluids

1.1.3 Viscoelastic fluids

Substances exhibiting characteristics of both ideal fluids and elastic solids and showing partial elastic recovery after deformation; These are categorizedas ‘visco-elastic fluids’, which are obtain by addition of small quantities of a high-molecular-weight polymer to a solvent result in a viscoelastic fluid,having both viscous and elastic properties.

1.2 Properties of non-Newtonian fluids

The non-Newtonian fluid properties with detailed explanation and examples are as shown in the table 1.1 the table gives details about the changes of viscosity property for different types of non-Newtonian fluids along with examples.

TABLE 1.1 Properties of non-Newtonian fluids

Type of the fluid Properties Examples

Generalized Newtonian fluids

Viscosity is constant. Stress depends on normal and shear strain rates and also the pressure applied on it Blood plasma, custard, water

Shear thinning (pseudo-plastic) Apparent viscositydecreases with increased stress Nail polish, whipped cream, ketchup, molasses, syrups, paper pulp in water

Shear thickening

(dilatant) Apparent viscosityincreases with increased stress Suspensions of corn starch in water, sand in