We begin this chapter with a quick review of the thermodynamics of the grand potential, which will be employed in this chapter to construct a phase field model, in contrast to the more traditional Helmholtz free energy (or entropy) typically employed in phase field modelling in the literature. The grand potential is defined by Ω = F − ∑ i = 1 n μ i N i , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003204312/34e3ad47-9f0c-49dc-acd7-5caec98ace28/content/math3_1.tif"/>