Examining two-component dynamics in supercritical fluids with inelastic X-ray scattering

Supercritical fluids (SCFs) represent a unique thermodynamic state with both liquid-like and gas-like thermophysical properties. Near the liquid-gas critical point, SCFs display structural heterogeneity due to the formation of molecular clusters. Unlike regular liquids, SCFs at the nanoscale exhibit two acoustic responses, regarded as two-component dynamics. While prior studies have observed two-component dynamics, the underlying physical mechanism of such an anomalous phenomenon remains unclear. In this study, we employ high-resolution inelastic X-ray scattering (IXS) to study the acoustic responses at sub-nanometer length scales in supercritical CO2. We present results from IXS measurements with energy resolution of 1.6 meV and momentum transfer range between 0.87 and 1.89 Å^-1. Measurements were performed across the Widom line at 88 bar for temperatures ranging between 292 K and 342 K. The measured current-current correlation function shows distinct low-frequency (LF) and high-frequency (HF) components. From the dispersion relations, it is shown that the HF component has liquid-like characteristics while the LF component follows an ideal-gas behavior. Complementing our experiments with molecular dynamics simulations reveals that the two-component dynamics emerges due to distinct momentum fluctuations from structural heterogeneities.