Role of “soft” wall confinement in the particle dynamics of supercritical fluids*

Supercritical fluids (SCF), having a heterogeneous phase behavior ("liquid-like" and "gas-like") across the Frenkel line (FL) [1] , seem to be a promising candidate for the study of the effect of wall softness on the particle dynamics. Recently, structural features of SCF have been found to vary significantly as the walls became “softer”[2].

Molecular dynamics simulations are carried out to understand the effect of softness of the walls on the particle dynamics of a supercritical fluid using mean-squared displacement (MSD) and the velocity autocorrelation function (VACF). Walls are systematically made “softer” by lowering the stiffness coefficient of the springs attached to each of the wall particles. The existence of non-diffusive modes, and the effect of wall softness on it, is investigated through the vibrational density of states (DoS). We discuss the contrasting trends observed in self-diffusion parallel to the walls, on the either side of the FL, as a function of the wall-stiffness. The effect of the “softness” of the walls to support or disrupt collective motion will also be discussed.


[1] V. V. Brazhkin, et al., Phys. Rev. Lett., 111, 145901, 2013.
[2] K. Ghosh, et al., Phys. Rev. E., 97, 012131, 2018.