Crossover between liquidlike and gaslike behavior in CH_{4} at 400 K

Area of Science:

• Thermodynamics
• Fluid Dynamics
• Materials Science

Background:

• Understanding fluid behavior across different states is crucial for chemical engineering and materials science.
• The Frenkel line is a theoretical concept describing a crossover in fluid dynamics, but experimental evidence has been limited.

Purpose of the Study:

• To provide experimental evidence for a crossover between liquidlike and gaslike states in fluid methane (CH4).
• To investigate the relationship between this crossover and the hypothesized Frenkel line.
• To characterize the crossover using spectroscopic and diffraction techniques.

Main Methods:

• Raman spectroscopy was used to analyze vibrational frequencies of methane.
• X-ray diffraction was employed to study the structural properties of methane.
• Experiments were conducted across a range of temperatures (up to 397 K) and pressures.

Main Results:

• A reversible crossover between liquidlike and gaslike states was observed in fluid methane.
• This crossover was detected up to 2.1 times the critical temperature of methane.
• Raman spectroscopy showed distinct pressure-dependent vibrational frequency behaviors in the liquidlike (increasing frequency) and gaslike (decreasing frequency) regions, consistent with the Frenkel line concept.

Conclusions:

• The experimental results support the existence of the Frenkel line as a crossover in fluid properties.
• The study demonstrates that fluid methane exhibits distinct liquidlike and gaslike dynamic behaviors.
• The findings have implications for understanding and modeling fluid behavior under extreme conditions.