Simple Materials

Abstract

In this chapter we are concerned with materials comprising a single type of elementary constituent, in the form of identical atoms or small molecules. These constituents exert forces on one another which decrease with the distance between them. When thermal agitation, which increases with the temperature of the system, is stronger than the interaction forces which tend on the whole to bring the constituent elements together, the matter is in the gaseous state. In this case, the mechanical properties of the material are associated with collisions between molecules and statistical tools can be used to obtain exact relations between the viscosity and the physical characteristics of the gas. When the attractive forces are strong enough compared with thermal agitation, the basic constituents tend to form a compact cluster. This is the liquid state. In this case, our understanding of the relationships between internal forces and the dynamical evolution of such disordered structures in flow remains incomplete. In certain cases, for example, at lower temperatures or higher pressures, the material may become slightly more compact than in the liquid state and arrange itself into an ordered structure. This is the solid state. The material can then be deformed to a certain extent and a direct relationship can be established between the force required and the local interaction forces.