Surface-directed spinodal decomposition in binary fluid mixtures on an amorphous wall: A molecular dynamics study

TL;DR

This molecular dynamics study investigates surface-directed spinodal decomposition in binary fluids near an amorphous wall, revealing distinct growth regimes, wetting layer dynamics, and supporting a hydrodynamic coarsening model.

Contribution

The paper provides detailed MD simulation insights into SDSD on amorphous walls, identifying two coarsening regimes and validating a hydrodynamic growth model with a crossover length scale.

Findings

Wetting layer thickness follows a power-law growth with two regimes: t^{1/3} and t^{1}.

Crossover time and length scale agree with theoretical predictions for bicontinuous domains.

No significant orientational effects on domain coarsening observed near the wall.

Abstract

We present molecular dynamics (MD) results to discuss wetting kinetics in binary fluid mixtures ($A:B=50:50$) undergoing surface-directed spinodal decomposition (SDSD) on an amorphous wall. Our simulations show the formation of a wetting layer rich in the preferred $A$-type particles and bicontinuous domain morphology in the bulk. In addition, the mixture maintains connectivity between the bulk and the wetting layer through $A$-rich tubes throughout the depletion region. The wetting layer thickness coarsens as a power law, $R_1(t)\sim t^{\alpha}$, with two distinct growth regimes of $\alpha=1/3$ and $\alpha=1$ active for at least a decade. The computed crossover time for $\alpha=1/3 \to 1$ equaled the reported bulk crossover time, and the corresponding crossover length scale $R_c$ agrees well with the expression $\Lambda = \sqrt{2k/\gamma_0}$ given by Scholten et al.~[\emph{Macromolecules}2005, 38, 3515] for bicontinuous domains in aqueous polymer mixtures in the presence of only one dominant length scale. This agreement supports a hydrodynamic picture of diffusive growth for the interconnected wetting layer and bulk domains, where the bending contribution ($k$) of curvature-dependent $AB$ interfacial tension ($\gamma$) governs small-scale coarsening, producing $t^{1/3}$ growth. For length scales beyond $\Lambda$, capillary flows yield the viscous hydrodynamic regime ($\sim t$). Our results show no orientational effects on the domain coarsening parallel and perpendicular to the wall, contrasting many continuum models, including combinations with Flory-Huggins theory.