  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.
