Onsager Principle-Based Domain Embedding for Thermodynamically Consistent Cahn-Hilliard Model in Arbitrary Domain

TL;DR

This paper introduces an Onsager principle-based domain embedding method to extend the Cahn-Hilliard model to arbitrary domains, ensuring thermodynamic consistency and enabling accurate numerical simulations.

Contribution

The paper develops a systematic domain embedding framework based on the Onsager principle to extend the Cahn-Hilliard model with thermodynamic consistency in arbitrary domains.

Findings

The extended model recovers the original Cahn-Hilliard model asymptotically.

The numerical scheme is structure-preserving and robust.

The method effectively handles gradient flow problems in complex geometries.

Abstract

The original Cahn-Hilliard model in an arbitrary domain with two prescribed boundary conditions is extended to a Cahn-Hilliard-type model in a larger, regular domain with homogeneous Neumann boundary conditions. The extension is based on the Onsager principle-based domain embedding (OPBDE) method, which has been developed as a systematic domain embedding framework to ensure thermodynamic consistency. By introducing a modified conservation law, the flux at the boundary of the original domain is incorporated into the conservation law as a source term. Our variational approach demonstrates that, even without a prior knowledge on the specific form of the rate of free energy pumped into the system, the Onsager principle remains an effective instrument in deriving the constitutive equation of the extended system. This approach clarifies the intrinsic structure of the extended model in the perspectives of free energy and its dissipation. Asymptotic analysis is carried out for the extended OPBDE Cahn-Hilliard model, demonstrating that the original Cahn-Hilliard model, including its boundary conditions, can be fully recovered. To validate our approach, a structure-preserving numerical scheme is developed to discretize the extended model. Numerical results show that the OPBDE Cahn-Hilliard model is accurate, effective, and robust, highlighting the capability of the OPBDE method in handling gradient flow problems in arbitrary domain geometries.