Structural behavior of supercritical fluids under confinement

Abstract

The existence of the Frenkel line in the supercritical regime of a Lennard-Jones (LJ) fluid shown through molecular dynamics (MD) simulations initially and later corroborated by experiments on argon opens up possibilities of understanding the structure and dynamics of supercritical fluids in general and of the Frenkel line in particular. The location of the Frenkel line, which demarcates two distinct physical states, liquidlike and gaslike within the supercritical regime, has been established through MD simulations of the velocity autocorrelation (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 Frenkel line through a series of MD simulations considering a set of thermodynamics states in the supercritical regime (P=5000 bar, 240K≤T≤1500K) 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 found to be distributed in distinct layers along the z axis with layer spacing less than one atomic diameter and the lateral RDF showing amorphous-like structure for specific spacings (packing frustration) and non-amorphous-like structure for other spacings. 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 confirms 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 normal gas regime. The heterogeneity across the Frenkel line, 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.

Frenkel line crossover of confined supercritical fluids

“…1). In other words, bulk SCF gradually develops its non-diffusive vibrational modes as we decrease the temperature from 600 K. FL crossover in bulk SCF is observed around 600 K at P = 5000 bar as the DoS completely loses vibrational component around 600 K. This is consistent with the complementary approach of determining FL crossover using VACF, as deduced in our previous work 7 . We note that while the peak in the DoS for corresponds to damped-oscillatory VACF (Fig.…”

“…We had seen earlier that partial confinement leads to intriguing evolution of particle dynamics and emerging structures at a fixed temperature far from any phase change 7,8 . We now look at the role of temperature on the dynamics and structure of partially confined SCF.…”

“…The spacing of 70 Å is chosen as a representative spacing close to the bulk as the particle distribution across the spacing exhibits identically flat distribution similar to the bulk for almost the whole spacing except the vicinity of the walls. The rest of the spacings are selected as strong structural inhomogeneities are found to occur at these spacings 7 .…”

“…In a recent experimental study of Frenkel line crossover in bulk supercritical Neon, C. Prescher and co-authors 6 found the width of this crossover to be smaller than 10% in both pressure and temperature. Our studies on partially confined supercritical LJ fluids 7,8 opened up the possibility of the existence of an amorphous phase in addition to the liquidlike and gaslike phases in supercritical LJ fluids. As partial confinement has been found to strongly influence the dynamics and induce significant changes in the structure, its role on the characteristics associated with the Frenkel line and its vicinity are worth investigating.…”

“…The wall atoms are attached to fcc lattice sites by harmonic springs. The spring constant (k) for these springs are fixed to a value of 1000 eV/Å 2 which mimic rigid walls 7,8 , by restricting the mean squared displacement (MSD) of the wall atoms with respect to their lattice sites to a very low value. The root mean squared displacement (RMSD) of wall particles are restricted to be 40 times and 100 times smaller than a typical distance traversed by a fluid particle between two collisions 1 (~1 Å), at 1500 K and 300 K respectively.…”