Molecular dynamics study of the evolution of heterogeneous structures in supercritical fluids

To understand the evolution of heterogeneous structures in supercritical fluids at the microscopic scale, this study performs molecular dynamics simulations on supercritical argon. First, the nearest-neighbor criterion is used to identify liquid-like and gas-like atoms, and the boundaries between the liquid-like, two-phase-like, and gas-like regions are determined based on the fraction of gas-like atoms. The clustering behavior in each phase region is then analyzed. As the temperature increases, the number of clusters first increases and then decreases, while both the maximum number of atoms in a cluster and the cluster size decrease monotonically. These trends indicate that supercritical fluids exhibit multiphase characteristics similar to subcritical gas-liquid coexistence. In addition, image processing techniques are employed to extract interface structures, and a concept of gas-liquid-like interface is proposed. This enables the quantitative characterization of the “specific pseudo-interface area.” Results show that the specific pseudo-interface area increases significantly when transitioning from the liquid-like region to the two-phase-like region, but decreases noticeably as the system evolves further into the gas-like region, peaking near the Widom line. This work provides theoretical insights into the properties of supercritical fluids.