Water molecules switch from quickstep to slowfox as they cool

“To understand water’s role as the solvent of life and as a regulator of Earth’s climate, we need to understand its anomalies, those properties that set it apart from other liquids,” says Robin Tyburski, researcher at the Department of physics, Stockholm University.

Deviating pattern

One long-standing question in water research concerns the behavior of supercooled water.

“When water is cooled below its freezing point of 0 °C, the motion of the molecules slows dramatically, making the liquid much more viscous. If you extrapolate that trend, the molecules would come to a complete standstill at -45 °C, meaning water would no longer behave as a liquid but as a solid. And yet, liquid water has been observed at temperatures as low as -137 °C,” says Anders Nilsson, professor at the Department of physics, Stockholm University..

Research breakthrough

This led the research team to test the hypothesis that there may be a transition point where this trend breaks, that is, a temperature at which the slowing of molecular motion becomes less pronounced. Until now, however, it has been impossible to investigate molecular motion at such low temperatures. When water is cooled this much, it freezes into ice in less than a millisecond, making direct observation previously impossible.

“By probing molecular motion in three steps, we were able to show that such a transition does indeed occur,” says Fivos Perakis, associate professor at the Department of physics, Stockholm University..

• First, the team prepared water at temperatures down to –45 °C by injecting tiny droplets (about 0.002 mm in diameter) into a vacuum chamber.
• Next, they used a laser pulse to induce a structural change in the water.
• Finally, they used X-ray pulses to track how, and how quickly, the molecular structure evolved.

Short timespan

All of this had to take place in under a millisecond, before the droplets froze. Only five facilities worldwide can generate X-ray pulses short and intense enough for such measurements. One of them is the SwissFEL free-electron laser in Switzerland, where the experiment was carried out.

“In this way, we could observe that structural changes in the molecules occur more than 20 times slower at –40 °C than at 0 °C. At even lower temperatures, however, the rate of structural change no longer slows as dramatically,” says Robin Tyburski.

Deepens understanding

The study also deepens our understanding of water’s dynamic properties at room temperature.

“One example is water’s unusually high heat capacity, which allows the oceans to regulate Earth’s climate. By measuring how molecules move, we now have a clearer picture of how water redistributes absorbed heat through structural rearrangements,” says Anders Nilsson.

The results open the door to further studies investigating in greater detail what happens to water’s structure around –40 °C.

“At this transition temperature, we also see another known change: below about –37 °C, water molecules appear to form slightly more stable structures than at higher temperatures. It would be exciting to determine whether this structural change is what drives the shift in how water molecules move,” says Robin Tyburski.

International collaboration

The study was conducted in collaboration with POSTECH University in South Korea, SwissFEL in Switzerland, and SACLA in Japan. Contributors from Stockholm University included Anita Girelli and Iason Andronis, as well as former colleagues Markus Soldemo, Maddalena Bin, and Kyung Hwan Kim.

Read more about the research group Experimental X-ray Studies of Liquids and Surfaces at the Department of Physics