Experimental evidence of non-equilibrium phase separation in supercritical fluids

Supercritical fluids (SCFs) have long been considered homogeneous and structureless, yet recent studies suggest the existence of transient, liquid-like clusters under dynamic processes. In this study, we provide experimental evidence of semi-stable non-equilibrium phase separation in SCFs through opacity measurements and small-angle neutron scattering (SANS). By investigating the thermophysical properties of helium, argon, and krypton during adiabatic expansion, we show that cooling dynamics vary significantly among species, influencing cluster formation. Neutron scattering measurements reveal distinct variations in signal intensity, supporting that the clusters slowly dissolve into the background with a surprisingly long time scale of tens of minutes. Given that SCFs in industrial applications frequently experience dynamic, non-equilibrium conditions rather than in strict thermodynamic equilibrium, our results provide crucial insights with potential implications for advanced material processing, energy systems, and chemical engineering.