Physicists from Stockholm University have achieved a scientific milestone by discovering a new critical point in water, solving a mystery that has puzzled researchers for over a century. Using cutting-edge technology, the team uncovered definitive evidence of a liquid-liquid critical point in supercooled water. This breakthrough explains some of the most bizarre and counterintuitive physical properties of Earth’s most vital resource.
The findings, published in the journal Science, mark the culmination of decades of theoretical debate. By utilizing ultrafast X-ray lasers at facilities in South Korea, the international research team pinpointed this elusive state at approximately -63 degrees Celsius under extreme pressures of 1,000 atmospheres. For years, scientists believed observing this critical point was physically impossible because water freezes into ice almost instantly under these exact conditions.
Decoding the Strange Physics of Water
Despite being essential for life, water behaves unlike almost any other liquid on the planet. Most substances shrink and become denser as they cool down. Water, however, reaches its maximum density at a relatively warm 4 degrees Celsius. As the temperature drops below this threshold, it begins to expand. This unique density anomaly is the reason why ice cubes float in your glass and why lakes freeze from the top down, protecting aquatic ecosystems during winter.
When water is supercooled—chilled below its standard freezing point without turning into solid ice—its behavior becomes even stranger. Instead of contracting, its volume continues to expand at an accelerating rate. Properties like heat capacity and compressibility also follow highly unusual trends as the temperature plummets.
Fundamentally, water can exist in two distinct macroscopic liquid phases. These phases differ based on how the water molecules form hydrogen bonds under varying temperatures and pressures. One phase features a low density with an open, tetrahedral network of bonds. The other phase is denser, characterized by a collapsed structure and distorted bonding.
Capturing a Fleeting Molecular Transition
At the newly discovered critical point, the boundary between these two distinct liquid phases completely vanishes. The two states merge into a single, highly unstable supercritical phase. This creates intense molecular fluctuations that ripple across a wide range of temperatures and pressures.
To capture this fleeting transition, researchers had to work on an unprecedented, ultrafast timescale. “What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how the liquid-liquid transition vanishes and a new critical state emerges,” said Anders Nilsson, a Professor of Chemical Physics at Stockholm University.
The experiments revealed that as water approaches this critical point, its molecular movement slows down dramatically. The system exhibits a glass-like behavior, becoming increasingly confined and destabilized. The research team compared this kinetic phenomenon to a gravitational black hole, creating a basin of no return where the molecular configuration of the water becomes trapped in endless fluctuations.
A Century-Old Debate Resolved
The search for this critical point builds on a century of scientific inquiry. X-ray pioneer Wolfgang Röntgen was among the first to speculate that water might exist in two different states. By 1992, a theoretical simulation published in Nature predicted the existence of the supercritical point. However, physically proving it remained out of reach until these recent advancements in ultra-short X-ray pulses.
“There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen,” Nilsson explained. “Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime.”
The Natural Pathway of Amorphous Ices
To access this notoriously difficult supercooled regime, the scientists studied amorphous ices. These are solid forms of water that lack a long-range, organized crystalline structure, allowing them to mimic liquid states under extreme conditions.
Aigerim Karina, a Postdoc in Chemical Physics at Stockholm University, noted how surprising this approach was. “It’s amazing how amorphous ices, such an extensively studied state of water, happened to become our entrance to the critical region,” Karina said. “It’s a great inspiration for my further studies and a reminder of the possibilities of making discoveries in much-studied topics such as water.”
The collaborative effort included researchers from POSTECH University and PAL-XFEL in South Korea, the Max Planck Society and Johannes Gutenberg University in Germany, and St. Francis Xavier University in Canada.
Implications for Life and Future Science
While the immediate findings resolve a major question in physical chemistry, the long-term implications span multiple disciplines. The fluctuations caused by this critical point are responsible for water’s dynamic instability and its ability to sustain diverse forms of life. In essence, ambient water operates as a supercritical fluid, constantly shifting between high-density and low-density arrangements.
Fivos Perakis, an associate professor in Chemical Physics at Stockholm University, highlighted the broader existential questions raised by the research. “I find it very exciting that water is the only supercritical liquid at ambient conditions where life exists, and we also know there is no life without water,” Perakis said. “Is this a pure coincidence, or is there some essential knowledge for us to gain in the future?”
Understanding the critical behavior of supercooled water could eventually revolutionize applied sciences. It opens new pathways for improving cryopreservation, controlling ice formation, and refining industrial processes that rely on the precise thermodynamic management of water.
