The Role of Entropy in the Specification of a Powder

The study of powders differs in a fundamental way from that of thermal systems in that friction dominates motion unless the powder is in a state of high agitation. In this chapter the high-agitation case is not considered and the much more general and widespread condition of powders at rest, or moving slowly, is studied. Whereas in a thermal system the motion ensured by temperature allows the system itself to seek that state of thermal equilibrium, a powder, as soon as external forces cease, rapidly loses its motion and comes to rest in a condition that reflects its history. The particular and central property is that the density at rest can vary. An arbitary powder can be compacted by applying pressure, and will not return to its initial density when the pressure is removed unless the grains of the powder possess elasticity. In this chapter attention will be confined to inelastic grains where there is strong friction between grains. A typical picture will be of powder gently poured into a container, where it sits under gravity (Fig. 4.1a). If a force F is applied it will compact (Fig. 4.1b) and retain this, when F is removed, to a smaller volume (Fig. 4.1c). If F is now increased the powder reaches a minimum volume where it will resist any further increase of the force (Fig. 4.1d).