First Principles

Before proceeding to the detailed study of the application of rheology to cheese, it is pertinent to consider a few basic principles. These will appear obvious to the reader educated in a physical science, but may be less obvious to those from other walks of life. One is taught that matter exists in one of three distinct states: gas, liquid, and solid. This context is mainly concerned with only the latter two. Both are characterized by the fact that any sample has a well-defined volume. Solids have a definite shape which can be altered only by the application of an external agency. Liquids, on the other hand, have no characteristic shape: they take up the shape of the vessel containing them.

The property, then, which distinguishes a solid is its rigidity and this term has been formally adopted by physicists as a measure of the effort required to change the sample's shape. The precise definition will be given later. If the material is a true solid, it will follow that this rigidity is invariant for the material. Furthermore, time does not enter into this, so it is understood that the change is instantaneous and once the effort is removed, the sample will recover its original shape spontaneously. This is, in plain language, the physicist's concept of an elastic solid.

In contrast with the solid, the characteristic by which a layman distinguishes a liquid is its fluidity. This, again, has a precise definition, but in practice it is more convenient to use the inverse concept, which is known as the vis cosity. This is a measure of the relation between the effort applied to the liquid and the rate at which it flows. As before, once the effort has been withdrawn, the flow ceases, but the liquid remains where it is—there is no recovery. It is the rate of flow which spontaneously reverts to its original value. This describes the physicist's concept of a viscous liquid.

It is important to keep clearly in mind that the fundamental difference in behavior between solids and liquids involves the dimension of time. Time does not enter at all into the description of the behavior of an elastic solid—only spatial dimensions are involved—whereas time is equally as important as the spatial dimensions in the description of fluid behavior.

So far, only ideal materials have been described. These are seldom encountered in practice and are only of interest to rheologists as reference points, the simplest extremes of material behavior, one may say the "black and white" of classical physics. It is that extensive "gray" area between that is the rheologist's domain. Everyone knows that a piece of cheese, once squeezed, may recover its original shape only slowly and probably not completely. Indeed, some of the softer cheeses may, if sufficient effort be applied, be spread and only recover very little. This leads to a definition of a third category of materials. Any material falling within this gray area, possessing at the same time some of the characteristics of elastic and of viscous behavior, is known as a viscoelastic material.

Yet another class of materials may be mentioned in passing, since they may be encountered in everyday parlance. This is the plastic material. Plasticity may be defined as that property of a material whereby it remains rigid until a certain minimum force is applied, whereupon it deforms. Once this force is removed or falls below the critical value, the material again becomes rigid but remains in the deformed state. There is a simple mathematical model for this type of behavior (5) and the rheologist may be particularly interested in the transition from solid to fluid behavior, known as the yield point; however, the ideal plastic is probably as rare as the ideal solid or liquid.

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