Kinetics of Phase Separation in Thin Films: Simulations for the Diffusive Case

TL;DR

This paper investigates the diffusion-driven phase separation in thin films, focusing on surface effects and metastable layered states, using Langevin simulations to analyze the crossover to domain coarsening.

Contribution

It provides new insights into the kinetics of surface-directed spinodal decomposition and the transition from layered states to coarsening in thin film geometries.

Findings

Metastable layered states form due to surface wetting effects.

Crossover from layered states to lateral coarsening is characterized.

Langevin simulations reveal the growth kinetics of domains.

Abstract

We study the diffusion-driven kinetics of phase separation of a symmetric binary mixture (AB), confined in a thin-film geometry between two parallel walls. We consider cases where (a) both walls preferentially attract the same component (A), and (b) one wall attracts A and the other wall attracts B (with the same strength). We focus on the interplay of phase separation and wetting at the walls, which is referred to as {\it surface-directed spinodal decomposition} (SDSD). The formation of SDSD waves at the two surfaces, with wave-vectors oriented perpendicular to them, often results in a metastable layered state (also referred to as ``stratified morphology''). This state is reminiscent of the situation where the thin film is still in the one-phase region but the surfaces are completely wet, and hence coated with thick wetting layers. This metastable state decays by spinodal fluctuations and crosses over to an asymptotic growth regime characterized by the lateral coarsening of pancake-like domains. These pancakes may or may not be coated by precursors of wetting layers. We use Langevin simulations to study this crossover and the growth kinetics in the asymptotic coarsening regime.