Abstract
According to textbook definitions1, there exists no physical observable able to distinguish a liquid from a gas beyond the critical point, and hence only a single fluid phase is defined. There are, however, some thermophysical quantities, having maxima that define a line emanating from the critical point, named ‘the Widom line’2 in the case of the constant-pressure specific heat. We determined the velocity of nanometric acoustic waves in supercritical fluid argon at high pressures by inelastic X-ray scattering and molecular dynamics simulations. Our study reveals a sharp transition on crossing the Widom line demonstrating how the supercritical region is actually divided into two regions that, although not connected by a first-order singularity, can be identified by different dynamical regimes: gas-like and liquid-like, reminiscent of the subcritical domains. These findings will pave the way to a deeper understanding of hot dense fluids, which are of paramount importance in fundamental and applied sciences.
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Acknowledgements
We acknowledge the ESRF for provision of beam time at ID28, and we thank D. Gambetti, M. Hoesch, J. Serrano-Gutierrez, A. Beraud and A. Bossak for fruitful discussions and assistance during the experiments. F.A.G. and M.S. have been supported by the European Community, under Contract No. RII3-CT2003-506350. T.S. has received financial support from the European Research Council under the European Community’s Seventh Framework Program (FP7/2007-2013)/ ERC grant Agreement No. 207916.
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F.A.G., M.S. and T.S. proposed the research, did the project planning, the experimental work, contributed to data analysis and interpretation, and wrote the paper. T.B. carried out the molecular dynamics simulations and contributed to data interpretation. G.G.S. and G.R. contributed to data analysis and interpretation, and to writing the paper. M.K. contributed to the experimental work and to data interpretation.
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Simeoni, G., Bryk, T., Gorelli, F. et al. The Widom line as the crossover between liquid-like and gas-like behaviour in supercritical fluids. Nature Phys 6, 503–507 (2010). https://doi.org/10.1038/nphys1683
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DOI: https://doi.org/10.1038/nphys1683
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