Molecular dynamics of partially confined Lennard-Jones gases: Velocity autocorrelation function, mean squared displacement, and collective excitations

Particle motion and correlations in fluids within confined domains promise to provide challenges and opportunities for experimental and theoretical studies. We report molecular dynamics simulations of a Lennard-Jones gas mimicking argon under partial confinement for a wide range of densities at a temperature of 300 K. The isotropic behavior of velocity autocorrelation function (VACF) and mean squared displacement (MSD), seen in the bulk, breaks down due to partial confinement. A distinct trend emerges in the VACF-perpendicular and MSD-perpendicular, corresponding to the confined direction, while the trends in VACF-parallel and MSD-parallel, corresponding to the other two unconfined directions are seen to be unaffected by the confinement. VACF-perpendicular displays a minimum, at short timescales, that correlates with the separation between the reflective walls. The effect of partial confinement on MSD-perpendicular is seen to manifest as a transition from diffusive to subdiffusive motion with the transition time correlating with the minimum in the VACF-perpendicular. When compared to the trends shown by MSD and VACF in the bulk, the MSD-perpendicular exhibits subdiffusive behavior and the VACF-perpendicular features rapid decay, suggesting that confinement suppresses the role of thermal fluctuations significantly. Repetitive wall-mediated collisions are identified to give rise to the minima in VACF-perpendicular and in turn a characteristic frequency in its frequency spectrum. The strong linear relation between the minima in VACF-perpendicular and wall-spacing suggests the existence of collective motion propagating at the speed of sound. These numerical experiments can offer interesting possibilities in the study of confined motion with observable consequences.