About the Phase Field Thin-Film Flow Method
The phase field thin-film flow method offers an attractive alternative to more established methods for solving multiphase flow problems in narrow channels represented by a surface or an edge within the geometry. Instead of directly tracking the interface between two fluids, the interfacial layer is governed by a phase field variable, ϕ. The surface tension force is added to the momentum equation as a body force by multiplying the chemical potential of the system by the gradient of the phase field variable.
The evolution of the phase field variable is governed by the Cahn–Hilliard equation, which is a 4th-order PDE. The Phase Field Thin-Film Flow interface decomposes the Cahn–Hilliard equation into two second-order PDEs.
For the level set method, the fluid interface is simply advected with the flow field. The Cahn–Hilliard equation, on the other hand, does not only convect the fluid interface, but it also ensures that the total energy of the system diminishes correctly. The phase field method thus includes more physics than the level set method.
The free energy of a system of two immiscible fluids consists of mixing, bulk distortion, and anchoring energy. For simple two-phase flows, only the mixing energy is retained, which results in a rather simple expression for the free energy.