Crossover between liquidlike and gaslike behavior in CH4 at 400 K

Abstract

We report experimental evidence for a crossover between a liquidlike state and a gaslike state in fluid methane (CH_{4}). This crossover is observed in all of our experiments, up to a temperature of 397 K, 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and x-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line-a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquidlike and gaslike properties. On the liquidlike side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gaslike side this competes with the attractive van der Waals potential leading the vibration frequency to decrease as pressure is increased.

Experimental and modeling evidence for structural crossover in supercritical CO2

“…The SFs were Fourier transformed to obtain the experimental PDF. As with previous experimental and modeling results on Ar [27], Ne [37], CH 4 [38], and, especially, H 2 O [39], pronounced changes in the first peak position in the PDF with temperature are observed, indicating a well-defined crossover. the first nearest-neighbor distance, r fnn [given by the radial position of the first peak in g(r)], and the system's "length" (V 1/3 ) to be proportional to each other unless the system undergoes a structural change.…”

“…Before concluding, we note that previous experiments detecting the structural crossover at the FL involved x-ray scattering in supercritical Ne [37], the combination of x-rays with Raman scattering in supercritical CH 4 [38], and the combination of neutron and Raman scattering in supercritical N 2 [49] . Only one small-angle neutron scattering experiment had been used to study the FL in CO 2 in the vicinity of the critical point only [50].…”

Excitation spectra in fluids: How to analyze them properly

“…1(a,e). The long-standing problem of the calculation of the q -gap has recently attracted considerable interest in view of crossover between liquid- and gas-like collective dynamics in fluids with change in temperature and pressure 8,26,32,34–45,47–58 . Here, we present the calculated excitation spectra near the q -gap and compare the dispersion relations obtained using the two-oscillator model to the results based on the analysis of transverse current spectra.…”

“…However, both these approaches have fundamental disadvantages, since anharmonicity effects are taken into account phenomenologically. Recently, excitation spectra in fluids have become of great interest in context of studies concerning the thermodynamics and collective dynamics of fluids 8,26,32–62 , and, in particular, crossover from fluid- to gas-like collective dynamics (Frenkel line) 8,32,34–41,47,48,50–53,56,58 and in the framework of inelastic X-ray or neutron scattering experiments 63–76 . Note that these experiments have already revealed a problem of mixing of longitudinal and transverse modes, which plays an important role in a proper analysis of excitation spectra as will be shown below.…”

Similarity between the kinematic viscosity of quark-gluon plasma and liquids at the viscosity minimum

“…Guided by theoretical prediction, subsequent experiments have confirmed the transition at the Frenkel line in supercritical Ne [52], CH 4 [53], N 2 [54], C 2 H 6 [55] and CO 2 [56] using X-ray, neutron and Raman scattering techniques.…”