Antidiffusion

Abstract

In Chap. 14 we have seen that the constitutive relation of the material flux of a chemical species in a mixture should be expressed in terms of the chemical potential gradient of that species. This statement can be easily justified considering the analogy between mass and heat transport: as thermal equilibrium is characterized by a uniform temperature, it is natural that when this condition is perturbed, the system will tend to return to its equilibrium state by inducing a thermal flux from hot to cold regions, that is proportional to the temperature gradient. Therefore, as chemical equilibrium is characterized by a uniform chemical potential, it is natural to assume that a system will tend to return to its equilibrium state by inducing a material flux that is proportional to the (negative) chemical potential gradient (see R. Mauri, Non-equilibrium Thermodynamics in Multiphase Flows, Springer, 2013). In this chapter, some of the consequences of this statement will be studied, starting in Sect. 21.1 with some elementary considerations about mixture thermodynamics. Then, in Sects. 21.2 and 21.3, we describe van der Waals’ theory of chemical stability and phase transition, applying these results to simple symmetric and regular binary mixtures in Sect. 21.4. Finally, in Sects. 21.5 and 21.6 diffusive fluxes are modeled in terms of chemical potential gradients.