Researchers Confirm Fragile-to-Strong Transition in Supercooled Water

A recent study published on December 14, 2025, in Nature Physics, marks a significant breakthrough in the understanding of water’s unique properties. Researchers have successfully observed a fragile-to-strong transition in deeply supercooled water for the first time, resolving a long-standing scientific enigma that has puzzled experts for nearly three decades.

Water exhibits anomalous behavior when cooled below its freezing point without crystallizing. While previous studies suggested that water’s viscosity would diverge to infinity around 227 K (–46°C), this prediction conflicted with established properties of water. To address these inconsistencies, scientists theorized that viscosity would undergo a fragile-to-strong transition (FST) at a specific low temperature.

Breaking New Ground with Innovative Techniques

Co-authors of the study, Professor Kyung Hwan Kim from POSTECH‘s Department of Chemistry and Professor Anders Nilsson from Stockholm University‘s Department of Physics, emphasized the importance of water in both life and natural phenomena. “The key to uncovering their origin is believed to lie in the deeply supercooled regime, an area where water freezes so rapidly that direct experimental investigation has been nearly impossible,” noted Kim.

Historically, direct observation of the fragile-to-strong transition had been elusive due to water’s rapid crystallization below approximately 235 K. The breakthrough came through the innovative combination of a droplet-based sampling technique and ultrafast X-ray free-electron lasers, enabling the researchers to create and analyze water droplets at deeply supercooled temperatures.

“We created liquid water down to –45°C by rapidly evaporating it under vacuum, allowing it to remain supercooled for only a brief moment,” explained Nilsson. This method involved generating micron-sized water droplets (about 17 μm across) and utilizing ultrashort X-ray pulses from the SwissFEL facility to track molecular motion with remarkable precision.

Monitoring Water’s Structural Relaxation

The research team employed femtosecond infrared laser pulses to induce minimal temperature changes in each droplet, monitoring how the liquid structure returned to equilibrium. By tracking the X-ray scattering signal’s response, they measured the relaxation rate of water and tested for a fundamental shift near the predicted transition.

The results indicated a clear dynamic crossover at approximately 233 K (around –40°C). Above this temperature, relaxation times increased rapidly, characteristic of fragile liquids. In contrast, below 233 K, the data exhibited a shallower temperature dependence typical of strong liquids.

Molecular dynamics simulations using the TIP4P/2005 water model corroborated the experimental findings, identifying a similar fragile-to-strong crossover around 238.7 K. “Fast crystallization makes experiments in this temperature regime challenging, so much of the previous work has been driven by computer simulations,” Kim stated, highlighting the importance of connecting experimental results with computational models.

The experimentally confirmed transition temperature of approximately 233 K lies slightly above the previously identified Widom line at 230 K, suggesting that the fragile-to-strong transition relates to changes in the populations of distinct molecular arrangements, rather than the glass transition at 136 K.

Implications and Future Research Directions

The implications of this study extend beyond fundamental physics. Understanding the dynamics of water can enhance knowledge in various fields, from weather patterns to biological chemistry. “By resolving the mystery behind water’s anomalous behavior, we can improve our understanding of many phenomena that depend on water,” Nilsson remarked.

The researchers believe this work opens new avenues for exploration. “We have not yet directly observed the detailed microscopic mechanisms behind this behavior,” Kim noted. He expressed optimism that further advancements would allow for experimental probing of these underlying mechanisms. Additionally, their method enables the study of water below 230 K, paving the way for investigating other phenomena within this temperature range.

This groundbreaking research not only confirms that water’s apparent divergence is interrupted by a real change in relaxation behavior but also reinforces decades of theoretical and simulation work on the fragile-to-strong crossover. The findings promise to deepen our understanding of water’s complex dynamics, fostering continued exploration in this vital area of science.