Densification of Supercritical Carbon Dioxide Accompanied by Droplet Formation When Passing the Widom Line

Thermal density fluctuations of supercritical ${\mathrm{CO}}_{2}$ were explored using small-angle neutron scattering (SANS), whose amplitude (susceptibility) and correlation length show the expected maximum at the Widom line. At low pressure, the susceptibility is in excellent agreement with the evaluated values on the basis of mass density measurements. At about 20 bar beyond the Widom line, SANS shows the formation of droplets accompanied by an enhanced number density of the supercritical fluid. The corresponding borderline is interpreted as a Frenkel line separating gas- and liquidlike regimes.