Introduction

The primary topic of this book is suspensions comprised of solid particles suspended in a liquid. When such materials are subjected to shear forces, the deformation is borne by the liquid phase, and it is in that phase and its interface with the particles that the flow causes energy to be dissipated. Therefore, the hydrodynamics of the liquid phase will always play a role in the rheology of a suspension, even in those cases where other phenomena, such as colloidal interparticle forces, contribute to the stresses. Therefore, a systematic study of the various parameters that govern suspension rheology starts with the hydrodynamic contribution, i.e., the contribution to the suspension stress that derives directly from the dissipation in the liquid phase of the suspension. As noted in the introduction, the flows of interest will be laminar and the particle Reynolds number will be sufficiently small that Stokes flow will be assumed, i.e., particle inertia will not be considered in the general treatment.

In suspensions of large, non-colloidal particles, i.e., with characteristic dimensions of a few micrometers or more, the contributions to the suspension stress from Brownian motion and from interparticle forces such as electrostatic interactions can often be ignored. Hence, such suspensions can be used to study hydrodynamic effects without interference from the other phenomena. However, because non-colloidal suspensions do not display Brownian motion, there is no diffusion to help generate an equilibrium structure. This causes some experimental and theoretical problems, as will be discussed. Therefore, some features are introduced in this chapter, to be elaborated on in Chapter 3 which explicitly treats Brownian motion. In the present chapter and in Chapters 3 and 4, only suspensions of spherical particles will be considered, in order to avoid at this stage the complexity introduced by shape effects. Non-spherical particles and the resulting shape effects will be treated in Chapter 5. Likewise, the complexities resulting from the use of non-Newtonian suspending media are not considered here. A number of important industrial suspensions, such as coatings and nanocomposites, can be based on polymeric media; these are covered in Chapter 10.