Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India.
J Chem Phys. 2019 Mar 21;150(11):111102. doi: 10.1063/1.5092121.
The interplay between the structure and dynamics of partially confined Lennard Jones (LJ) fluids, deep into the supercritical phase, is studied over a wide range of densities in the context of the Frenkel line (FL), which separates rigid liquidlike and non-rigid gaslike regimes in the phase diagram of the supercritical fluids. Extensive molecular dynamics simulations carried out at the two ends of the FL (P = 5000 bars, T = 300 K, and T = 1500 K) reveal intriguing features in supercritical fluids as a function of stiffness of the partially confining atomistic walls. The liquidlike regime of a LJ fluid (P = 5000 bars, T = 300 K), mimicking argon, partially confined between walls separated by 10 Å along the z-axis, and otherwise unconstrained, reveals amorphous and liquidlike structural signatures in the radial distribution function parallel to the walls and enhanced self-diffusion as the wall stiffness is decreased. In sharp contrast, in the gas-like regime (P = 5000 bars, T = 1500 K), soft walls lead to increasing structural order hindering self-diffusion. Furthermore, the correlations between the structure and self-diffusion are found to be well captured by excess entropy. The rich behaviour shown by supercritical fluids under partial confinement, even with simple interatomic potentials, is found to be fairly independent of hydrophilicity and hydrophobicity. The study identifies persisting sub-diffusive features over intermediate time scales, emerging from the strong interplay between density and confinement, to dictate the evolution and stabilization of structures. It is anticipated that these results may help gain a better understanding of the behaviour of partially confined complex fluids found in nature.
部分受限 Lennard-Jones（LJ）流体的结构与动力学相互作用，深入到超临界相，在超临界流体相图中分离刚性液态和非刚性气态区域的 Frenkel 线（FL）的广泛密度范围内进行研究。在 FL 的两端（P = 5000 巴，T = 300 K 和 T = 1500 K）进行的广泛分子动力学模拟揭示了超临界流体作为部分约束原子壁刚性的函数的有趣特征。LJ 流体的液态（P = 5000 巴，T = 300 K），模拟氩气，在沿着 z 轴分离 10 Å 的壁之间部分受限，并且不受其他限制，在平行于壁的径向分布函数中显示出无定形和液态结构特征，并增强了自扩散，随着壁刚性的降低。相比之下，在气态（P = 5000 巴，T = 1500 K）中，软壁导致结构有序性增加，阻碍了自扩散。此外，发现结构和自扩散之间的相关性很好地由过剩熵来捕获。即使使用简单的原子间势，超临界流体在部分受限下表现出的丰富行为也与亲水性和疏水性相当独立。该研究确定了在中间时间尺度上持续存在的亚扩散特征，这些特征源自密度和约束之间的强烈相互作用，从而决定了结构的演化和稳定。预计这些结果可能有助于更好地理解自然界中部分受限的复杂流体的行为。
