Crossover between liquid-like and gas-like behaviour in CH4 at 400 K

D. Smith; M. A. Hakeem; P. Parisiades; H. E. Maynard-Casely; D. Foster; D. Eden; D. J. Bull; A. R. L. Marshall; A. M. Adawi; R. Howie; A. Sapelkin; V. V. Brazhkin; J. E. Proctor

doi:10.1103/physreve.96.052113

arXiv · 1710.04212

Crossover between liquid-like and gas-like behaviour in CH4 at 400 K

D. Smith·M. A. Hakeem·P. Parisiades·H. E. Maynard-Casely·D. Foster·D. Eden·D. J. Bull·A. R. L. Marshall·A. M. Adawi·R. Howie·A. Sapelkin·V. V. Brazhkin·J. E. Proctor

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Abstract

We report experimental evidence for a crossover between a liquid-like state and a gas-like state in fluid methane (CH4). This crossover is observed in all of our experiments, up to 397 K temperature; 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 liquid-like and gas-like properties. On the liquid-like 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 gas-like side this competes with the attractive Van der Waals potential leading the vibration frequency to decrease as pressure is increased.

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D. Smith, M. A. Hakeem, P. Parisiades, H. E. Maynard-Casely, D. Foster, D. Eden, D. J. Bull, A. R. L. Marshall, A. M. Adawi, R. Howie, A. Sapelkin, V. V. Brazhkin, J. E. Proctor. 2017-10-23. Crossover between liquid-like and gas-like behaviour in CH4 at 400 K. https://doi.org/10.1103/physreve.96.052113

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