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.