Liquid-vapor interface of a polydisperse fluid

TL;DR

This study uses Grand Canonical Monte Carlo simulations to analyze the liquid-vapor interface of a polydisperse fluid, revealing size segregation effects and challenges in measuring interfacial width due to polydispersity.

Contribution

It provides the first detailed simulation-based analysis of size segregation and interfacial properties in polydisperse fluids, confirming theoretical predictions.

Findings

Smaller particles segregate to the interface.

Polydispersity influences interfacial width measurements.

Size distribution varies between coexisting phases.

Abstract

We report a Grand Canonical Monte Carlo simulation study of the liquid-vapor interface of a model fluid exhibiting polydispersity in terms of the particle size $\sigma$. The bulk density distribution, $\rho^0(\sigma)$, of the system is controlled by the imposed chemical potential distribution $\mu(\sigma)$. We choose the latter such that $\rho^0(\sigma)$ assumes a Schulz form with associated degree of polydispersity $\approx 14%$. By introducing a smooth attractive wall, a planar liquid-vapor interface is formed for bulk state points within the region of liquid-vapor coexistence. Owing to fractionation, the pure liquid phase is enriched in large particles, with respect to the coexisting vapor. We investigate how the spatial non-uniformity of the density near the liquid-vapor interface affects the evolution of the local distribution of particle sizes between the limiting pure phase forms. We find (as previously predicted by density functional theory, Bellier-Castella {\em et al}, Phys. Rev. {\bf E65}, 021503 (2002)) a segregation of smaller particles to the interface. The magnitude of this effect is quantified for various $\sigma$ via measurements of the relative adsorption. Additionally, we consider the utility of various estimators for the interfacial width and highlight the difficulties of isolating the intrinsic contribution of polydispersity to this width.