What separates a liquid from a gas?

According to current understanding, no differences can be made between a gas and a liquid above the critical point, an abrupt terminus of the liquid-gas coexisting line.

Recently, it was reported that this is not the case, and that the phase diagram of matter should be modified. In their Physics Today article, Vadim Brazhkin from the Institute for High Pressure Physics in Moscow and Kostya Trachenko from Queen Mary University in London report on their discovery of a new line on the phase diagram.

They have proposed that a new line ("Frenkel line") exists on the phase diagram above the critical point at arbitrarily high pressure and temperature, and which separates two physically distinct states of matter.

Crossing the line corresponds to qualitative changes of the key physical properties of the system. As a result, the system properties cross over from liquid-like to gas-like. Above the line, shear resistance of the system is lost at all available frequencies. Consequently, specific heat becomes 2kB, corresponding to the potential energy of shear modes becoming zero. Viscosity, the speed of sound, and thermal conductivity all decrease with increasing temperature below the Frenkel line, as in liquids, but increase with temperature sufficiently above the line, as in gases. The behavior of the diffusion constant crosses over from exponential temperature dependence below the Frenkel line, as in liquids, to power-law dependence above the line, as in gases. And crossing the Frenkel line also results in the disappearance of so-called high-frequency fast sound and roton minima.

Supercritical fluids, including H2O and CO2, have recently started to be widely deployed in important environmental and industrial applications. Apart from fundamental theoretical importance, this discovery will assist in wider application and optimization of existing supercritical fluids and will drive the search for the new ones.

Click here for the link to the paper in Physics Today:

http://www.physicstoday.org/resource/1/phtoad/v65/i11/p68_s1?bypassSSO=1

Journal link: V. V. Brazhkin, Yu. D. Fomin, A. G. Lyapin, V. N. Ryzhov and K. Trachenko, Two liquid states of matter: A new dynamic line on a phase diagram, Physical Review E 85, 031203 (2012), PDF

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