Widom line

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. 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.

Other proposed similar boundary lines include for example the Fisher-Widom line and the Frenkel line.

Overview
The Widom line has been suggested to separate liquid-like behaviour and gas-like behaviour in supercritical fluids, where traditional phase boundaries between liquid and gas no longer exist. Specifically, the supercritical fluid has gas-like behavior on the low-pressure side of the Widom line and liquid-like behavior on the high-pressure side of the Widom line. This separation is a continuous crossover in some of the properties of the fluid and has been observed in laboratory experiments, for example on fluid methane. 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 and ongoing research area.

The Widom line is named after the theoretical physicist Benjamin Widom.