# Asymptotic Behaviour of Time Stepping Methods for Phase Field Models

## TL;DR

This paper analyzes the asymptotic behavior of various time stepping methods for phase field models like Allen Cahn and Cahn Hilliard, predicting optimal step sizes for metastable dynamics and confirming findings through computational experiments.

## Contribution

It provides a formal asymptotic analysis of different time stepping methods, including energy stable and implicit schemes, for phase field models during metastable dynamics.

## Key findings

- Energy stable methods often require more steps than others.
- Backward Euler preserves energy decay and profile fidelity at larger steps.
- Eyre type methods perform worse due to loss of profile fidelity.

## Abstract

Adaptive time stepping methods for metastable dynamics of the Allen Cahn and Cahn Hilliard equations are investigated in the spatially continuous, semi-discrete setting. We analyse the performance of a number of first and second order methods, formally predicting step sizes required to satisfy specified local truncation error $\sigma$ in the limit of small order parameter $\epsilon \rightarrow 0$ during meta-stable dynamics. The formal predictions are made under stability assumptions that include the preservation of the asymptotic structure of the diffuse interface, a concept we call profile fidelity. In this setting, definite statements about the relative behaviour of time stepping methods can be made. Some methods, including all so-called energy stable methods but also some fully implicit methods, require asymptotically more time steps than others.The formal analysis is confirmed in computational studies. We observe that some provably energy stable methods popular in the literature perform worse than some more standard schemes. We show further that when Backward Euler is applied to meta-stable Allen Cahn dynamics, the energy decay and profile fidelity properties for these discretizations are preserved for much larger time steps than previous analysis would suggest. The results are established asymptotically for general interfaces, with a rigorous proof for radial interfaces. It is shown analytically and computationally that for most reaction terms, Eyre type time stepping performs asymptotically worse due to loss of profile fidelity.