The Widom line for supercritical fluids

We review the different approaches to establish the boundary between the liquid and the quasi-gas fluid above the critical point. Some experimental data by inelastic X-ray scattering demonstrates different dynamical regimes: gas-like and liquid-like reminiscent of the subcritical domains. Theoretical models based on the Lennard-Jones and square well interparticle potentials together with phenomenological Van der Waals model confirm similar picture for the Widom line in supercritical domain. To minimize uncertainty experimental data we calculated extrema of response functions via fundamental equations of state. The shift of critical points under influences of impurities and nanoparticle adding is analyzed.