Chemical Potential Under Nonhydrostatic Stress
The purpose of this chapter is to continue the unification that was begun in Chapter 8. There, first and second derivatives of normal stress with respect to orientation were used; we now examine the idea that the chemical potential of a component at a point can be a multivalued direction-dependent scalar like the normal-stress magnitude, and that it too can have a gradient with respect to orientation. The essence of a nonhydrostatic stress is that different planes through a point are subject to different normal compressive stresses: σn varies with the orientation of the plane considered. Let us focus on a plane i across which the normal compressive stress is σi: then we put forward the assertion that an equilibrium state that can be associated with plane i is a hydrostatic state whose pressure has the same magnitude as σi. For illustration, see Figure 9.1. (For the present, we take a cautious stance: each hydrostatic state in the figure is certainly an equilibrium state, and each is certainly associated with a plane, but is it the associated equilibrium state that properly belongs with that plane according to the precepts of, for example, de Groot (1951, p. 11)? For now, we make no attempt to prove that it is so: we simply use the assertion and explore its consequences. Fortunately its consequences include large amounts of classical mechanics so that it counts as a "successful assertion" on those grounds, but at least for now it lacks any underpinnings.) An immediate consequence of the assertion illustrated in Figure 9.1 is the relation in Figure 9.2.