In the context of the pressure-temperature phase diagram of a substance and of the supercritical fluid state in particular, the Widom line is a line emanating from the critical point which in a way extends the liquid-vapor coexistence curve above the critical point. It corresponds to the maxima or minima of certain physical properties of the supercritical fluid, such as the speed of sound, isothermal compressibility, isochoric and isobaric heat capacities. A common criterion for locating the Widom line is indeed the maximum in the isobaric heat capacity.
More generally, the Widom line is defined as the line in the pressure-temperature phase diagram of a fluid substance along which the correlation length has its maximum.[1] It always emanates from a critical point. It has been investigated for various systems, including for example in the context of the hypothesized liquid–liquid critical point (or second critical point) of water.[2]
Similar boundary lines include the Fisher-Widom line and the Frenkel line, which also describe transitions between distinct fluid behaviors.
Overview
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Named after theoretical physicist Benjamin Widom, the Widom line is a crucial concept in fluid thermodynamics and critical phenomena.
The Widom line has been suggested[3] to separate liquid-like behaviour and gas-like behaviour in supercritical fluids, where the traditional distinction between liquid and gas no longer exists. Specifically, on the low-pressure side of the line, the fluid exhibits a gas-like behavior, while on the high-pressure side, it behaves more like a liquid. This separation is not a sharp phase change but a continuous crossover in some of the properties of the fluid. It has been observed in laboratory experiments, for example on fluid methane.[4] The concept of Widom line provides a useful framework for characterizing and predicting the properties of fluids, which are important for scientific research as well as various industrial processes. Such a concept is indeed relevant to the physical properties of any single-component fluid at sufficiently high pressures and temperatures, and its study is an active research area.
See also
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edit- ^ H. E. Stanley, P. Kumar, G. Franzese, L. Xu, Z. Yan, M. G. Mazza, S. V. Buldyrev, S.-H. Chen and F. Mallamace "Liquid polyamorphism: Possible relation to the anomalous behaviour of water", European Physical Journal - Special Topics 161 pp. 1-17 (2008)
- ^ Limei Xu, Pradeep Kumar, S. V. Buldyrev, S.-H. Chen, P. H. Poole, F. Sciortino, and H. E. Stanley "Relation between the Widom line and the dynamic crossover in systems with a liquid–liquid phase transition", PNAS 102 pp. 16558-16562 (2005)
- ^ Simeoni, Giovanna Giulia; Bryk, Taras; Gorelli, Federico Aiace; Krisch, Michael; Ruocco, Giancarlo; Santoro, Mario; Scopigno, Tullio (2010). "The Widom line as the crossover between liquid-like and gas-like behaviour in supercritical fluids". Nature Physics. 6 (7): 503–507. Bibcode:2010NatPh...6..503S. doi:10.1038/nphys1683. ISSN 1745-2473.
- ^ U. Ranieri, F. Formisano, F. A. Gorelli, M. Santoro, M. M. Koza, A. De Francesco, L. E. Bove "Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid", Nature Communications 15, 4142 (2024)