Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid

Area of Science:

• Physics
• Chemical Engineering
• Materials Science

Background:

• Supercritical fluids lack distinct liquid-gas phases above the critical point.
• Transition boundaries like Widom and Frenkel lines are proposed for supercritical states.
• Understanding supercritical fluid dynamics is crucial for applications.

Purpose of the Study:

• To investigate the dynamic behavior of supercritical methane.
• To identify crossovers between gas-like and liquid-like dynamics.
• To explore implications for supercritical fluid physics and applications.

Main Methods:

• Incoherent quasi-elastic neutron scattering (IQuENS) measurements.
• Analysis of self-dynamic structure factor.
• Fitting neutron spectra with Gaussian and Lorentzian functions.

Main Results:

• Observed a crossover from Gaussian (gas-like) to Lorentzian (liquid-like) self-dynamics.
• Crossover occurred around the Widom line (59 bar) at 200 K.
• Liquid-like jump diffusion mechanism described behavior at higher pressures, across the Frenkel line.

Conclusions:

• Supercritical fluid methane exhibits a progressive gas-like to liquid-like dynamic crossover.
• This confirms complex physics in supercritical states, challenging textbook definitions.
• Findings have potential planet-wide implications and applications in green chemistry.