Critical Opalescence

As mentioned above, at the critical point the correlation length diverges so that the correlation function obeys a power-law. This of course implies that the system has no characteristic length at this point and is scale-invariant, exhibiting fluctuations of all length scales. Consider liquid water heated in a sealed container. As the temperature is raised, more of the liquid will vaporise, quickly increasing the density of the gas phase. At the same time the density of the liquid phase decreases, so that eventually a point is reached (about 647K and 218 atm) where the two phases have the same density. Near this point one finds in the fluid domains of liquid-like and vapour-like phases of all sizes (that is, scale-invariant fluctuations between the two phases). In particular once there are liquid drops of the same size as the wavelength of visible light, strong scattering takes place giving the fluid a cloudy appearance: this is called critical opalescence. Experimentally it is easier to observe critical opalescense in binary fluid mixtures that show a second-order phase transition with regard to their mixing. See pictures in Ref[11].