Diffuse Interface Stress Tensor

We show that under equilibrium conditions, an anisotropic phase-eld model of a diiuse interface leads to the notion of a divergence-free tensor, whose components are negligible away from the interface. Near the interface, it plays the r^ ole of a stress tensor, and involves the phase-eld generalisation of the Cahn-Hooman-vector. We show that this tensor may be used to derive the equilibrium conditions for edges in interfaces. We then extend these ideas from the phase-eld model to a general class of multiple-order-parameter models. In this broader context we demonstrate that it is also possible to construct a-vector, and we use it to derive the three-dimensional form of the Gibbs-Thomson equation in the sharp-interface limit. We derive the associated stress tensor for the multiple-order-parameter case, and show that it also leads to the appropriate equilibrium conditions for edges in interfaces and multiple junctions in the sharp-interface limit. This approach shows that both multiple-order-parameter models and less physically-based phase-eld models share a common framework, the former providing a generalisation of the notion of the-vector for phase-eld models as well as providing a connection to the classical-vector theory for a sharp interface. For the special case of a phase-eld model with non-convex surface energy, we show that edges can be represented by weak shocks in which the spatial derivatives of the phase eld are not continuous, and we derive the associated jump conditions. In all the situations considered the notion of the-vector and the stress tensor play a central r^ ole.