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arXiv · 1811.07046

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

Kanka Ghosh·C. V. Krishnamurthy
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Abstract

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.

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BibTeXRIS

Kanka Ghosh, C. V. Krishnamurthy. 2018-11-15. Molecular dynamics of partially confined Lennard-Jones gases: Velocity autocorrelation function, mean squared displacement, and collective excitations. https://doi.org/10.1103/physreve.98.052115

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