Research

[POSTECH Observes Nonequilibrium Phase Separation, Verifies Nanoclusters Persisting for Over an Hour]

 A supercritical fluid^*1 refers to a state in which the temperature and pressure of a substance exceed its critical point, where no distinction exists between liquid and gas phases. Traditionally, it has been regarded as a single, uniform phase. However, a research team at POSTECH (Pohang University of Science and Technology) experimentally demonstrated nonequilibrium phase separation within supercritical fluids by observing nanometer-sized “liquid clusters” that persist for up to one hour.

 The research team led by Professor Gunsu Yun from the Division of Advanced Nuclear Engineering and the Department of Physics at POSTECH, in collaboration with Dr. Jong Dae Jang’s group at the Korea Atomic Energy Research Institute (KAERI), Professor Min Young Ha at Kyung Hee University, and Dr. Changwoo Do’s team at Oak Ridge National Laboratory (ORNL) in the U.S., experimentally verified the existence of nano-clusters that exist separately in a liquid-like state within supercritical fluids previously considered a uniform phases. The experiment utilized the Small-Angle Neutron Scattering (SANS)^*2 instrument at Korea’s neutron research facility, HANARO.

 Supercritical fluids, appearing when temperature and pressure exceed the critical point, have long been understood as a uniform state without phase separation, where the boundary between liquid and gas disappears. Recent simulation studies have suggested that, under equilibrium conditions (with constant temperature, pressure, and concentration), supercritical fluids may contain sub-regions resembling “gas-like” and “liquid-like” states. However, less is known for the possbility of phase separation in supercritical fluids under nonequilibrium conditions, that are very common  in industrial applications  where pressure and temperature change rapidly.

 In this study, the team compressed krypton gas under high pressure to generate a supercritical fluid and closely observed changes in neutron scattering signals over time. They confirmed the presence of clusters averaging 1.3 nanometers in size with liquid-like properties—roughly corresponding to aggregates of about 30 krypton atoms. Remarkably, these clusters persisted for over an hour before disappearing. This discovery overturns the prevailing notion that supercritical fluids exist only as a single phase, providing the first experimental evidence that dynamic environments can give rise to phase separation phenomena.

 Diagnosing and controlling this nonequilibrium phase separation could enable more precise design and control of supercritical fluid processes. In practice, most industrial uses of supercritical fluids involve nonequilibrium flow conditions. Under such conditions, tiny liquid clusters can significantly affect cleaning efficiency, solubility, and heat transfer. These insights have important implications for a wide range of industries, including semiconductor cleaning, pharmaceutical manufacturing, food processing, thermal-fluid systems in power plants, and rocket engine development. Furthermore, the findings may also help explain similar fluid phenomena in natural extreme environments, such as the supercritical atmosphere of Venus or the high-temperature, high-pressure fluids within Earth’s crust.

 Professor Yun remarked, “This is the experimental verification of phase separation of nano-sized liquidlike clusters in nonequilibrium supercritical fluids, which was first conjectured several years ago (see Lee et al., "Quasi-equilibrium phase coexistence in single-component supercritical fluids", Nature Comm. 12 (2021) 4630).  Our findings not only provide insight to industrial process optimization but also to understanding extreme natural environments such as atmospheres of gas giants and subsurface fluids within the Earth.”

 The study has been published in Communications Physics, an international journal in the field of physics, and was supported by the National Research Foundation of Korea.

DOI: https://doi.org/10.1038/s42005-025-02263-2

1. supercritical fluid: A fluid that has reached a temperature and pressure beyond its critical point. The critical point varies depending on the type of fluid; for krypton, used in this study, the critical point is about –60 °C and 55 bar. Supercritical fluids are characterized by low viscosity, very high thermal conductivity, and strong chemical reactivity, making them useful in a wide range of industrial applications.

2. small-angle neutron scattering, SANS: An experimental technique in which a sample is irradiated with neutrons, and the signals scattered at very small angles are measured to analyze nanometer-scale structures and material properties.