Structural behavior of supercritical fluids under confinement

TL;DR

This study investigates how supercritical Lennard-Jones fluids behave structurally under partial confinement, revealing layering and ordering effects that vary with wall rigidity and regime, impacting the structure-dynamics relationship.

Contribution

It provides new insights into the structural changes of supercritical fluids under confinement across the Frenkel line, highlighting the effects of wall rigidity and confinement type.

Findings

Layering and increased order with wall rigidity

Distinct layering patterns for atomistic and reflective walls

Heterogeneity affects structure-dynamics scaling

Abstract

The location of the Frenkel line (FL) of a Lennard-Jones fluid which demarcates two distinct physical states, liquidlike and gaslike within the supercritical regime, has been established through molecular dynamics (MD) simulations of the velocity auto-correlation (VACF) and Radial distribution Function (RDF). We, in this article, explore the changes in the structural features of supercritical LJ-fluid under partial confinement using atomistic walls. The study is carried out across the FL through a series of MD simulations considering a set of thermodynamics states in the supercritical regime of Argon well above the critical point. Confinement is partial, with atomistic walls located normal to z and extending to "infinity" along the x and y directions. In the "liquidlike" regime of the supercritical phase, particles are distributed in distinct layers along the z with layer spacing less than one atomic diameter and the lateral RDF showing amorphous-like structure for specific spacings (packing frustration). Increasing the rigidity of the atomistic walls is found to lead to stronger layering and increased structural order. For confinement with reflective walls, layers are found to form with one atomic diameter spacing and the lateral RDF showing close-packed structure for the smaller confinements. Translational order parameter and excess entropy assessment confirm the ordering taking place for atomistic wall and reflective wall confinements. In the "gaslike" regime of the supercritical phase, particle distribution along the spacing and the lateral RDF exhibit features not significantly different from that due to the normal gas regime. The heterogeneity across FL, found to be present both in bulk and confined systems, might cause the breakdown of the universal scaling between structure and dynamics of fluids necessitating the determination of a unique relationship between them.