Nonlinear dynamics of phase separation in ultra-thin films

TL;DR

This paper develops a mathematical model for phase separation in ultra-thin films, analyzing long-term behavior without rupture, and provides existence and regularity results for the equations.

Contribution

It introduces a long-wavelength approximation of the Navier-Stokes Cahn-Hilliard equations with a regularising potential, and proves existence and regularity of solutions.

Findings

Established a nonzero lower bound for film height preventing rupture

Compared theoretical bounds with numerical simulations

Demonstrated the model's capability to describe coupled surface and phase dynamics

Abstract

We present a long-wavelength approximation to the Navier-Stokes Cahn-Hilliard equations to describe phase separation in thin films. The equations we derive underscore the coupled behaviour of free-surface variations and phase separation. Since we are interested in the long-time behaviour of the phase-separating fluid, we restrict our attention to films that do not rupture. To do this, we introduce a regularising Van der Waals potential. We analyse the resulting fourth-order equations by constructing a solution as the limit of a Galerkin approximation, and obtain existence and regularity results. In our analysis, we find a nonzero lower bound for the height of the film, which precludes the possibility of rupture. The lower bound depends on the parameters of the problem, and we compare this dependence with numerical simulations. We find that while the theoretical lower bound is crucial to the construction of a smooth, unique solution to the PDEs, it is not sufficiently sharp to represent accurately the parametric dependence of the observed dips in free-surface height.