Thermodynamic crossovers in supercritical fluids.

Area of Science:

• Thermodynamics
• Statistical Mechanics
• Condensed Matter Physics

Background:

• Beyond the critical point, supercritical fluids are conventionally defined as a single phase.
• Recent studies suggest dynamical crossovers indicating distinct liquid-like and gas-like behaviors in supercritical fluids.
• Existing theoretical models often propose a single crossover line, which is debated.

Purpose of the Study:

• To investigate the existence and nature of distinct states within supercritical fluids.
• To reconcile theoretical inconsistencies regarding supercritical fluid behavior and the Ising model.
• To propose a new framework defining boundaries of liquid-like, indistinguishable, and gas-like states in supercritical fluids.

Main Methods:

• Analysis of the supercritical behavior of the Ising model.
• Theoretical calculations and modeling of thermodynamic properties.
• Examination of experimental data, including inelastic X-ray scattering and small-angle neutron scattering.

Main Results:

• Identified two distinct thermodynamic crossover lines in supercritical fluids.
• Demonstrated that these lines follow critical scalings consistent with the Ising universality class near the critical point.
• Validated the upper crossover line with experimental data for supercritical argon and carbon dioxide.
• Verified the lower crossover line using equations of state for the compressibility factor.

Conclusions:

• The existence of two crossover lines provides a more accurate description of supercritical fluid states.
• This framework reconciles the Ising model's behavior with experimental observations.
• Offers a fundamental understanding of supercritical physics and phase transitions.