Ph.D. Candidate: Peihao Sun
Research Advisor: Jerome Hastings
Date: Friday, February 26, 2021
Time: 2:00 PM PST
Zoom Link: https://stanford.zoom.us/j/94828316014
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Title: Molecular Dynamics of Supercritical Fluids
Supercritical fluids have received renewed interest in the past few decades because of their industrial applications. Underlying many of these applications is the high tunability of the fluid properties in the near-critical region, which is not captured by the widely held notion of the supercritical state as a single broad continuum. In fact, recent works show that the supercritical region can be divided into liquid-like and gas-like states based on thermodynamic and transport properties with a rapid crossover at the Widom line (WL). However, the picture is less clear when it comes to the microscopic dynamics. While some works find that the WL is also associated with a dynamic transition in the supercritical state, others finds the dynamic transition at a different location, which in the case of water can be as much as 150 K away near the critical pressure.
This discrepancy reveals a lack of experimental data and physical understanding of the molecular-scale dynamics in the supercritical state. Therefore, my dissertation work has focused on this problem. The dissertation contains two main parts. In the first part, a combination of inelastic X-ray scattering measurements and molecular dynamics (MD) simulations is used to study supercritical water. The results show that, contrary to commonly used models, there exist two components in the intermolecular dynamics, and it is the ratiobetween the two that changes rapidly at the WL and drives the liquid-like to gas-like transition. In the second part, three additional systems---silicon, tellurium, and the Lennard-Jones fluid---are investigated via MD simulations, and the two-component behavior is found to be universal. The fraction of the liquid-like component is quantified and found to correlate with the degree of intermolecular bonding. The consequences of the two-component phenomenon for modeling supercritical fluid properties will be discussed.