# Soft-wall induced structure and dynamics of partially confined supercritical fluids

## TL;DR

This study investigates how the structure and dynamics of supercritical Lennard-Jones fluids are affected by partial confinement and wall stiffness, revealing complex behaviors linked to the Frenkel line and excess entropy.

## Contribution

It provides new insights into the interplay between structure, dynamics, and confinement in supercritical fluids across different regimes, using molecular dynamics simulations.

## Key findings

- Soft walls increase structural order in the liquidlike regime.
- In the gaslike regime, soft walls hinder self-diffusion despite increased order.
- Structural and dynamical behaviors are well correlated with excess entropy.

## Abstract

The interplay between the structure and dynamics of partially confined Lennard Jones (LJ) fluids, deep into the supercritical phase, is studied over a wide range of densities in the context of the Frenkel line (FL), which separates rigid liquidlike and non-rigid gaslike regimes in the phase diagram of the supercritical fluids. Extensive molecular dynamics simulations carried out at the two ends of the FL (P = 5000 bars, T = 300 K, and T = 1500 K) reveal intriguing features in supercritical fluids as a function of stiffness of the partially confining atomistic walls. The liquidlike regime of a LJ fluid (P = 5000 bars, T = 300 K), mimicking argon, partially confined between walls separated by 10 {\AA} along the z-axis, and otherwise unconstrained, reveals amorphous and liquidlike structural signatures in the radial distribution function parallel to the walls and enhanced self-diffusion as the wall stiffness is decreased. In sharp contrast, in the gas-like regime (P = 5000 bars, T = 1500 K), soft walls lead to increasing structural order hindering self-diffusion. Furthermore, the correlations between the structure and self-diffusion are found to be well captured by excess entropy. The rich behavior shown by supercritical fluids under partial confinement, even with simple interatomic potentials, is found to be fairly independent of hydrophilicity and hydrophobicity. The study identifies persisting sub-diffusive features over intermediate time scales, emerging from the strong interplay between density and confinement, to dictate the evolution and stabilization of structures. It is anticipated that these results may help gain a better understanding of the behavior of partially confined complex fluids found in nature.