Molecular dynamics simulation and nonlinear analysis of density fluctuations in Lennard-Jones fluid system near the critical point

Y Wang and JL Xu and QY Wang, CHINESE JOURNAL OF PHYSICS, 84, 132-151 (2023).

DOI: 10.1016/j.cjph.2022.12.012

Spatiotemporal density is important to understand the fluid behavior especially near the critical point (NCP). Here, molecular dynamics (MD) was applied to investigate Lennard-Jones (LJ) fluid NCP. Periodic boundary conditions are applied on the six surfaces of a simulation box (SB) consisting of 10976 LJ fluid atoms. A slice of 3.45a is selected in the SB, where a is the length scale of the LJ fluid. Our results show that the time dependent densities in the slice obviously deviate from the average value in SB. The deviations are found to display M-shape distribution with increases of average densities. The maximum deviation occurs at 0.8pc instead of at pc, where pc is the density at the critical point. Attention is paid on the phase distribution in the SB. Three regimes of liquid-like, two-phase-like and gas-like are observed to evolve with time. The density variation is explained by the combined effect of potential induced mechanism and critical fluctuation mechanism. The time series of densities in the slice behave either chaotic or random characteristics. The sample entropy in chaotic system is smaller than that in random system. Our work is helpful to understand the density variation and phase distribution in simple LJ fluid system when crossing the critical point.

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