Drink in this factoid: water is the strangest liquid of all.

Most fluids have predictable and similar behaviors. But unlike other fluids, water is more dense in liquid form, not solid. Aquatic life survives the winter because the ice floats instead of sinking and expanding into a huge solid glacier. The unique yet strange properties of water help support life.

For decades, scientists have tried to figure out what is going on with the strange behavior of water. The answers seem to lie in a long hidden window of extreme temperatures.

In 2020, scientists from Pacific Northwest National Laboratory (PNNL) made a giant leap in understanding the phenomenon. Detailed in the journal Science, the team used a revolutionary laser heating technique which revealed for the first time-The nanoscale changes that supercooled liquid water undergoes between -117.7 ° F (190 K) and -18.7 ° F (245 K).

The technique has pulled the curtain back from that previously enveloped temperature window where the weird and subtle structural changes in water occur. Greg Kimmel, PNNL chemical physicist described this unexplored expanse as “the whole ball game to understand the structure of water”.

This ball game is part of the Condensed Phase and Interfacial Molecular Sciences program sponsored by the Office of Basic Energy Science of the US Department of Energy. The program funds research to understand the fundamental physics and chemistry of systems that are far from equilibrium, and how they come to equilibrium. In this case, this system consists of liquids, in particular water.

“Water is one of the most important solvents we have,” Kimmel said. “We are trying to better understand how water behaves at interfaces, in confinement and in solutions, how it condenses and crystallizes, etc.

The implications are far-reaching, ranging from biological and physical processes related to climate change, to better chemistries for energy and nuclear processing, to new drugs to fight disease.

Researchers from all these fields will soon come together PNNL Energy Science Center, scheduled to open in late 2021. The new 140,000 square foot site will accommodate up to 250 theorists, experimenters, visiting scientists and support staff, not to mention the latest scientific instruments. Kimmel and his colleagues look forward to working in a collaborative environment while remaining focused on supercooled water.

Physicist Greg Kimmel and physical chemist Loni Kringle. (Photo by Andrea Starr | Pacific Northwest National Laboratory)

A question of balance or not

“When you lower the temperature, most liquid molecules clump together very tightly and are very dense. But below 39 ° F, water is just the opposite, ”explained Loni Kringle, who worked as a postdoctoral researcher with Kimmel’s team on supercooled water studies. “Water molecules form tetrahedral bonds that take up a lot of space. As the water cools, it expands and decreases in density. Think of the ice cubes coming out of their tray.

Scientists understand this big picture very well, but how does it go in detail? Not really.

Water that remains in liquid form well below the normal freezing point, called supercooled water, is far from true equilibrium, the most stable state. If its structure does not change, the water is in a so-called metastable state. The experiments of Kimmel and his team measured the rate at which supercooled water relaxes from its starting configuration to “metastable equilibrium” before crystallizing.

“Whether you want your material to reach equilibrium or not depends on what properties you want it to have,” Kimmel explained, using radioactive waste as an example. “If you want to capture and retain radioactive nuclei, you want to conserve glass, not crystalline material, which can grow grains and push impurities from the surface. It would be a problem. “

Pulsed laser test
To study supercooled water, thousands of laser pulses move a sample of water toward crystallization inside an ultra-vacuum chamber. Stop-motion images of the spectra during the first 30 pulses show water existing simultaneously in high and low density reversible structures. (Facilitated by Stephanie King | National Laboratory of the Pacific Northwest)

From burping waste to supercooled water

Kimmel joined PNNL in 1992 to study the reactions responsible for the accumulation and sudden release of hydrogen gas from nuclear waste stored in underground tanks at the DOE’s Hanford site. He simulated the “burping” process by projecting electrons onto thin layers of water.

His work aligned well with that of his scientific colleague from PNNL Bruce Kay’s research on the structure and kinetics of films at interfaces, examining how water desorbs and releases energy over a range of temperatures. The two scientists pursued the idea of ​​trying laser heating to measure the rate at which water crystallizes and diffuses.

Theories existed about reversible structural transformations before water crystallized, at temperatures above -171 ° F (160 K) and below -36 ° F (235 K) – but there was no evidence. Previous experiences jumped just over time.

“This temperature range is very difficult to reach and control experimentally, and that’s what the pulsed heating technique has overcome,” explained Kringle. She worked alongside fellow postdoctoral researcher Wyatt Thornley to perform the experiments and help analyze the data.

The team’s follow-up research, published in the Proceedings of the National Academy of Sciences in April, looked at “the details of the kinetics – how the films of water relax into two structural patterns,” Kringle said. “We looked at the specifics of the structural changes, going beyond qualitative observations by calculating the differences starting from high temperatures versus low temperatures, and then comparing the results with models in the literature. “

Close-up photo of the PNNL ultra-vacuum chamber.
Inside an ultra-vacuum chamber, a sample of water is exposed to laser pulses that last only about 10 nanoseconds each. (Photo by Andrea Starr | Pacific Northwest National Laboratory)

New research directions

Going forward, the team plans to work with Valeria Molinero, a professor at the University of Utah, to better understand the kinetics and dynamics occurring during pulsed heating experiments. Molinero is an expert in molecular dynamics simulations of aqueous systems.

Collaborations like this embody the vision behind the Energy Sciences Center. The researchers are already thinking about the different directions that the new place and their pulsed heating technique could take, them and others.

One idea is to change the temperature of their experiment before the water reaches the metastable equilibrium state. This adjustment would allow them to study how water “remembers” and “ages”, as seen in supercooled glass research.

Another avenue of study consists in examining “heavy water” which contains deuterium, a natural isotope of hydrogen. Deuterium contains an extra neutron which makes it heavier than a standard hydrogen atom. Comparing the quantum-scale interactions that occur in heavy water versus regular water will give scientists more clarity on the odd behavior of water versus other liquids.

And because pulsed laser heating lends itself to rapid reactions, other researchers have expressed interest in using the technique for chemistry studies.

Meanwhile, Kringle has his own plans.

“The timescales of our technique have been a limitation when looking at pure water. I did a quick exploratory experiment and found that if we add other molecules to the water, like carbon monoxide, we can change the temperature where the structural transition occurs, ”Kringle said. “I would like to follow up and see what happens at the end of the transition. This will provide information on the solubility of the other molecules that we add.

Kringle, who is also passionate about STEM Education and Awareness, is now a permanent scientist, joining Kimmel and Kay in PNNL Physical Sciences Division, chaired by Wendy Shaw.

“Loni is a prime example of the next generation of scientists and engineers who will carry the baton of scientific discovery into the future, not only at PNNL and the new Energy Science Center, but at research institutes across the country.” Shaw said.



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