In the context of the temperature-pressure 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 somehow extends the liquid-vapor coexistence curve above the critical point. It corresponds to the maxima or minima in 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 the line in the phase diagram of a fluid substance along which the correlation length has its maximum.[1] It always emanates from a critical point and has been also discussed for example in the context of the hypothesized liquid–liquid critical point (or second critical point) of water.[2]
Other proposed similar boundary lines include for example the Fisher-Widom line and the Frenkel line.
- ^ 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)