By subjecting water chilled to temperatures below its freezing point to investigation with high-speed lasers prior to its solidification, physicists affiliated with Stockholm University have detected definitive evidence of a long-hypothesized transition between two distinct liquid phases, characterized by significant surges in heat capacity and critical fluctuations.
You et al. investigated supercooled water on timescales preceding ice formation by heating amorphous ices of both high and low density, employing infrared ultrafast laser pulses, subsequently followed by X-ray scattering; a swift escalation in heat capacity was observed, signaling a critical divergence at 210 K, coinciding with amplified density fluctuations. Image attribution: POSTECH University.
The remarkable aspect, according to Professor Anders Nilsson of Stockholm University, was the ability to perform X-ray analysis at extremely rapid speeds before ice formation occurred, thereby enabling observation of how the transition between liquid states dissipates and a novel critical state emerges.
For several decades, speculation and various hypotheses have attempted to elucidate these exceptional properties, with one prominent theory positing the existence of a critical point. Current findings now confirm the presence of such a point.
Through the utilization of X-ray lasers, Professor Nilsson and his research team successfully ascertained the existence of a critical point within supercooled water, situated at approximately 210 Kelvin (equivalent to -63 degrees Celsius or -81 degrees Fahrenheit) and under pressures of 1,000 atmospheres.
Water exhibits unique characteristics, allowing it to exist in two distinct macroscopic liquid phases that feature differing molecular bonding arrangements at low temperatures and elevated pressures, as the researchers elucidated.
As temperature rises and pressure diminishes, a specific state is reached where the distinction between these two liquid phases dissolves, resulting in the presence of only a single phase.
This nexus represents a state of significant instability, inducing fluctuations across an extensive spectrum of temperature and pressure, extending even to ambient conditions.
The water molecules oscillate between the two liquid states and their mixtures, as if unable to commit to a definitive arrangement. It is these dynamic fluctuations that impart water with its unusual characteristics.
The condition that exists beyond a critical point is termed supercritical, and ambient water resides within this state.
An additional noteworthy discovery from this research is the observation that the system’s dynamics decelerate as it approaches the critical point.
Dr. Robin Tyburski from Stockholm University remarked that it appears almost impossible to escape the critical point once entered, akin to a black hole.
Dr. Aigerim Karina, a postdoctoral researcher at Stockholm University, expressed her astonishment at how amorphous ices, a widely studied form of water, served as the entry point to the critical region.
She further stated that this discovery serves as a significant inspiration for her ongoing research and a potent reminder of the potential for groundbreaking discoveries within extensively investigated subjects like water.
Iason Andronis, a Ph.D. student at Stockholm University, shared that achieving the ability to measure water under such extremely low temperatures without it freezing was a dream realized.
He commented that numerous individuals had aspired to locate this critical point, but the necessary technological means were unavailable until the advent of X-ray lasers.
Dr. Fivos Perakis of Stockholm University finds it profoundly exciting that water is the sole supercritical liquid found at ambient conditions where life thrives, underscoring the indispensable role of water in its existence.
He posed the question of whether this is a mere coincidence or if there is fundamental knowledge yet to be gained.
Professor Nilsson noted that an intense debate has persisted regarding the origins of water’s peculiar properties for over a century, dating back to the foundational work of Wolfgang Röntgen.
He added that researchers delving into the physics of water can now confidently adopt the model asserting that water possesses a critical point within its supercooled regime.
The subsequent phase of this research will involve ascertaining the implications of these findings on water’s crucial role in physical, chemical, biological, geological, and climate-related processes, presenting a substantial challenge for the coming years.
These particular discoveries were formally presented on March 26 in the esteemed journal, Science.
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Seonju You et al. 2026. Experimental evidence of a liquid-liquid critical point in supercooled water. Science 391 (6792): 1387-1391; doi: 10.1126/science.aec0018
