In a state that defies classical definitions, matter reveals behaviors we are only just beginning to understand.
Imagine a substance that can effuse through solid materials like a gas while simultaneously dissolving compounds like a liquid. This is not a material from science fiction but a supercritical fluid, a unique state of matter that exists beyond the familiar boundaries of liquid and gas.
For decades, scientists believed these fluids were uniform and predictable. However, recent groundbreaking discoveries have revealed a hidden complexity that is reshaping everything from clean energy production to pharmaceutical manufacturing. This article explores the fascinating science behind supercritical fluids and the new experiments that are challenging long-held assumptions.
A supercritical fluid forms when a substance is heated and pressurized beyond its critical point—a specific threshold of temperature and pressure where the distinction between liquid and gas phases disappears7 .
At this point, the fluid can no longer be condensed into a liquid by increasing pressure, nor can it be transformed into a gas by raising the temperature.
In this state, matter acquires hybrid properties: it possesses the penetrating ability of a gas, allowing it to effuse through porous solids, combined with the dissolving power of a liquid7 . Perhaps most remarkably, supercritical fluids lack surface tension, as there is no liquid/gas phase boundary to create it7 .
Visualization of the critical point where liquid and gas phases become indistinguishable
Several substances are commonly used as supercritical fluids in industrial and laboratory processes. Their critical properties vary significantly, making different fluids suitable for different applications.
| Solvent | Critical Temperature (°C) | Critical Pressure (atm) | Critical Density (g/cm³) |
|---|---|---|---|
| Carbon dioxide (CO₂) | 31.0 | 73.8 | 0.469 |
| Water (H₂O) | 374.0 | 217.7 | 0.322 |
| Ethane (C₂H₆) | 32.2 | 48.1 | 0.203 |
| Ethanol (C₂H₅OH) | 240.8 | 60.6 | 0.276 |
Source: 7
Carbon dioxide is particularly popular in industrial applications due to its accessible critical point, low toxicity, and low cost7 . Supercritical CO₂ has become the preferred method for decaffeinating coffee and extracting floral fragrances from flowers, replacing potentially harmful organic solvents7 .
For years, the textbook description of supercritical fluids portrayed them as perfectly uniform phases without any internal structure. This conventional understanding was recently upended by a team of researchers at POSTECH (Pohang University of Science and Technology) in collaboration with several institutions including Oak Ridge National Laboratory2 .
In October 2025, Professor Gunsu Yun and his team published experimental evidence revealing that supercritical fluids are not always uniform but can contain nanometer-sized "liquid clusters" under certain conditions2 . Their methodology was ingenious:
The researchers compressed krypton gas under high pressure to generate a supercritical fluid2 .
They used a Small-Angle Neutron Scattering (SANS) instrument at Korea's neutron research facility, HANARO, to observe changes in neutron scattering signals over time2 .
This specialized equipment allowed them to detect structures within the fluid that were invisible to conventional observation methods.
The experiment focused specifically on nonequilibrium conditions—situations where temperature and pressure change rapidly, which are actually more common than static conditions in industrial applications2 .
The research team made several remarkable discoveries:
They confirmed the presence of liquid-like clusters averaging just 1.3 nanometers in size—roughly corresponding to aggregates of about 30 krypton atoms2 .
These clusters were not fleeting; they persisted for over an hour before disappearing2 .
This finding provided the first experimental evidence that dynamic environments can give rise to phase separation phenomena in supercritical fluids2 .
This discovery has profound implications. Since most industrial applications of supercritical fluids involve nonequilibrium flow conditions, the presence of these tiny liquid clusters could significantly affect cleaning efficiency, solubility, and heat transfer in processes ranging from semiconductor manufacturing to pharmaceutical production2 .
"Our findings not only provide insight into industrial process optimization but also to understanding extreme natural environments such as atmospheres of gas giants and subsurface fluids within Earth"
Research into supercritical fluids requires specialized equipment designed to withstand extreme pressures and temperatures while allowing precise control and measurement.
Contains fluid under supercritical conditions. Withstands extreme pressure/temperature; optical or neutron access for measurement5 .
Detects nanostructures within fluids. Uses neutron scattering to reveal internal structures2 .
Separates compounds using SC fluids. Pressure up to 10,000 psi; precise temperature control3 .
The complexity of this equipment explains why certain properties of supercritical fluids remained hidden for so long. As demonstrated by the POSTECH team's research, advancing our understanding often requires both innovative thinking and specialized technology2 .
In a fascinating scientific twist, even as the POSTECH team was discovering liquid clusters in supercritical krypton, researchers at Ruhr University Bochum in Germany were reaching a very different conclusion about supercritical water5 .
In March 2025, a team led by Martina Havenith and Dominik Marx used a combination of terahertz spectroscopy and molecular dynamics simulations to study the structure of supercritical water5 .
Their experimental setup required overcoming significant technical challenges, including designing a specialized high-pressure cell large enough for terahertz spectroscopy yet strong enough to withstand extreme conditions5 .
The German team's results directly contradicted the cluster theory for water:
This contradiction suggests that different supercritical fluids may behave differently, or that the methods used (neutron scattering versus terahertz spectroscopy) may be detecting different phenomena. Clearly, the final chapter on supercritical fluid structure has not yet been written.
Supercritical fluids are not merely laboratory curiosities—they occur naturally in some of Earth's most extreme environments and throughout our solar system.
Researchers from the University of Tokyo have recently documented how supercritical water influences seismic activity and volcanic eruptions4 . Using advanced seismic imaging and machine learning, Professor Takeshi Tsuji and his team mapped how these deep fluids become trapped, migrate, and undergo phase changes that influence earthquakes4 .
Their study, published in September 2025, identified a continuous horizontal reflector that acts as a low-permeability seal confining supercritical fluids, along with fractures that disrupt this seal, creating "permeable windows" for fluid migration8 . These findings not only help explain earthquake mechanisms but also assist in identifying optimal sites for tapping into supercritical geothermal energy—a potentially vast renewable resource4 .
| Environment | Supercritical Fluid | Significance |
|---|---|---|
| Deep-sea hydrothermal vents | Water | Supports unique ecosystems; influences ocean chemistry |
| Volcanic zones | Water | Affects earthquake activity; potential geothermal energy source |
| Gas giant atmospheres (Jupiter, Saturn) | Hydrogen, helium | Influences planetary weather patterns |
| Venusian atmosphere | Carbon dioxide | Contributes to extreme greenhouse effect |
The atmospheres of gas giants like Jupiter and Saturn contain supercritical fluids, as does the atmosphere of Venus7 . Understanding the behavior of supercritical fluids in these environments helps planetary scientists model atmospheric dynamics and climate patterns on other worlds.
The unique properties of supercritical fluids have made them invaluable across numerous industries:
Supercritical fluid technology has "attracted enormous interest from researchers over the traditional pharmaceutical manufacturing strategies due to the environmentally benign nature and economically promising character of SCFs". The Solution-Enhanced Dispersion by Supercritical Fluids (SEDS) process is particularly effective for creating micro- and nano-scale drug particles with enhanced bioavailability.
Supercritical water oxidation is used to destroy hazardous wastes by completely breaking down organic compounds into harmless carbon dioxide, water, and salts.
Besides decaffeination, supercritical CO₂ is used to extract hops for beer production, spices for cooking, and essential oils for fragrances—all without leaving behind toxic solvent residues.
The science of supercritical fluids continues to evolve rapidly, with recent discoveries challenging fundamental assumptions about these unique states of matter. The conflicting findings regarding molecular clusters in different supercritical fluids suggest that we are still unraveling the complexities of this fourth state of matter.
What remains clear is that supercritical fluids represent a fascinating frontier in material science—one that bridges fundamental physics and practical applications across countless industries. As research continues, particularly in exploring nonequilibrium conditions that mirror real-world applications, our understanding of these remarkable fluids will undoubtedly continue to transform, offering new possibilities for green chemistry, clean energy, and advanced manufacturing.
The next time you drink a cup of decaffeinated coffee or hear about geothermal energy breakthroughs, remember the extraordinary state of matter that makes it possible—a substance that is neither liquid nor gas, but something mysteriously in between.