1.1 Introduction

This book is concerned with the physical behavior of a collection of small particles in fluids and with the motions they can sustain, particularly sound waves. By a small particle, we mean a material body of small dimensions, and made of a gas (bubble), a liquid (droplet), or a solid. Roughly speaking, the particle size range we will be interested in is between one and a few hundred microns. The combination of many such particles and a fluid is referred to as a suspension.

Examples of suspensions of particles in fluids abound, whether we are aware of them or not. Thus, the majestic beauty of a cumulus cloud is due to both its size and to its large number of droplets; the murmurs of a brook are due to their small bubbles; and the colors of paints and inks, both natural and man-made, are due to small particles. Fogs, smokes, dusts, salad dressings, milk, blood, and many other familiar examples are also suspensions. This list makes it evident that suspensions are important in many fields, both basic and applied. For example, the settling of small particles is important in both cloud physics and in the manufacture of paints. Particle settling, or sedimentation, is but one out of many motions sustained by suspensions. This book is concerned with a subset of those motions, namely sound waves. Although apparently limited in scope, this subset includes heat transfer, mechanics, thermodynamics, and, of course, acoustics.

Before delving into these matters...