Stable Equilibria of Anisotropic Particles on Substrates: a Generalized Winterbottom Construction

TL;DR

This paper introduces a generalized Winterbottom construction to predict stable equilibrium shapes of crystalline islands on substrates, accounting for anisotropic surface energies and multiple stable configurations.

Contribution

It extends the classical Winterbottom construction to handle strongly anisotropic and cusped surface energies, identifying all possible stable equilibrium shapes.

Findings

Multiple stable island shapes can exist with strong anisotropy.

The generalized construction accurately predicts stable shapes.

Dynamical evolution confirms the stability of predicted shapes.

Abstract

We present a new approach for predicting stable equilibrium shapes of crystalline islands on flat substrates, as commonly occur through solid-state dewetting of thin films. The new theory is a generalization of the widely used Winterbottom construction i.e., an extension of the Wulff construction for particles on substrates). This approach is equally applicable to cases where the crystal surface energy is isotropic, weakly anisotropic, strongly anisotropic and "cusped". We demonstrate that, unlike in the classical Winterbottom approach, multiple equilibrium island shapes may be possible when the surface energy is strongly anisotropic. We analyze these shapes through perturbation analysis, by calculating the first and second variations of the total free energy functional with respect to contact locations and island shape. Based on this analysis, we find the necessary conditions for stable equilibria and exploit this through a generalization of the Winterbottom construction to identify all possible stable equilibrium shapes. Finally, we propose a dynamical evolution method based on surface diffusion mass transport to determine whether all of the stable equilibrium shapes are dynamically accessible. Applying this approach, we demonstrate that islands with different initial shapes may evolve into different stationary shapes and show that these dynamically-determined stationary states correspond to the predicted stable equilibrium shapes, as obtained from the generalized Winterbottom construction.