Atomistic scale simulations are a powerful tool to study supercritical fluids and can provide information that is not accessible experimentally. The first part of this talk is an introduction to molecular dynamics (MD) simulations using interatomic potentials, commonly referred to as force fields. The second part of the talk presents results from MD simulations of supercritical solid-liquid phase transitions in nanoconfined ice and supercritical liquid-vapor transitions in binary mixtures.
Phase transitions in nanoconfined ice: We investigate the phase transitions of nanoconfined water in 2D graphene nanocapillaries and 1D carbon nanotubes using MD simulations in search for a solid-liquid critical point. We observe three regimes along the solid-liquid phase transition line:
(i) a sub-critical regime at low densities in which a first-order phase transition separates the solid and liquid phases
(ii) a continuous-transition regime at intermediate densities in which a solid-liquid continuous transition line that is characterized by a crossover in diffusivity separates the solid and liquid-like regions
(iii) a single phase regime at high densities where thermodynamic, structural and transport properties cannot distinguish the solid and liquid-like phases.
Widom line in binary mixtures: The second part of the talk is on supercritical liquid-vapor transitions in binary mixtures. Recent experiments have identified distinct liquid-like and vapor-like regimes in pure fluids under supercritical conditions. The supercritical liquid-vapor transition in a pure fluid occurs across an extension to the coexistence line, marked by almost discontinuously changing fluid properties. Nishikawa and Tanaka first identified this line, called the Widom Line, experimentally. Here, we perform MD simulations on binary Ar-Kr and Ne-Kr mixtures and present evidence for the existence of the Widom Line in binary mixtures. Interestingly, the Ar-Kr and Ne-Kr mixtures transition from a liquid-like to gas-like regime differently via distinct structural and dynamic pathways. The presence of distinct phase-transitions even in these ‘simple’ monoatomic binary mixtures highlights the complexity to be expected in higher order mixtures in real-life applications.