TABLE OF CONTENTS A simple substance such as water below its freezing point is a hard three-dimensional crystalline solid, and above its freezing point it is a low-viscosity Newtonian liquid. In the liquid state, the mechanical properties of such a substance are specified by its shear viscosity ?, which is of course temperature- and pressure-dependent.
It has been known for a century or more, however, that some condensed-phase materials are neither simple liquids nor simple crystalline solids, and thus they do not fall readily within the classical scheme of materials classification. These "complex fluids" possess mechanical properties that are intermediate between ordinary liquids and ordinary solids. Specification of a viscosity or an elastic modulus does not even begin to describe the mechanical properties of such a substance. In many cases, the relationship between stress and deformation for a complex fluid is nonlinear, is unknown, or is under dispute. The number of recognizably different kinds of complex fluids has gradually increased throughout this century, so that it is now possible to find materials that possess to an intermediate degree almost any of the properties that distinguish classical solids and liquids.
Perhaps the most important distinction between classical solids and classical liquids is that the latter quickly shape themselves to the container in which they reside, while the former maintain their shape indefinitely. Many complex fluids are intermediate between solid and liquid in that while...
