Rapid decompression key to making low-density liquid water

February 13, 2018, Carnegie Institution for Science
An illustration shows how rapid decompression is the key to observing low-density liquid water. Low density water mediates the rapid decompression transition from ice-VIII to ice Ic. Credit: Chuanlong Lin and Guoyin Shen

Water makes up more than 70 percent of our planet and up to 60 percent of our bodies.

Water is so common that we take it for granted. Yet water also has very strange properties compared to most other liquids. Its solid is less dense than its liquid form, which is why ice floats; its peculiar heat capacity profile has a profound impact on ocean currents and climate; and it can remain liquid at extremely cold temperatures.

In addition to ordinary water and water vapor, or steam, there are at least 17 forms of water ice, and two proposed forms of super-cooled liquid water.

New work from Carnegie high-pressure geophysicists Chuanlong Lin, Jesse Smith, Stanislav Sinogeikin, and Guoyin Shen found evidence of the long-theorized, difficult-to-see low-density liquid of water. Their work is published by Proceedings of the National Academy of Sciences.

The normal density of water is one gram of water molecules per each cubic centimeter. Studies of anomalies in water's behavior have indicated the existence of liquid water with both lower and higher densities than this standard. But observing these phenomena experimentally has been difficult.

Each molecule has what's called a phase diagram—a sort of chart indicating how its bulk molecular structure changes form under different temperature and pressure conditions. The parts of the phase diagram where low-density water is thought to occur are notoriously difficult to explore, the so-called "water's no-man's land," because they require a path through a series of very specific, very difficult conditions.

But the Carnegie team was able to observe low-density water as an intermediate phase using a newly developed rapid-decompression technique to turn the high-pressure crystalline phase ice-VIII to the diamond-like ice Ic at temperatures between about -207 and -163 degrees Fahrenheit (140 and 165 kelvin).

Sophisticated x-ray analysis confirmed the observation of the low-density liquid water phase, which only lasted for about half a second at -163 degrees Fahrenheit (160 kelvin).

When ice-VIII was decompressed at moderate speeds, it formed other phases of ice, indicating that the speed of decompression is key to observing the low-density liquid water phase.

"Our newly developed, very fast decompression method was the key to this exciting observation of low-density liquid as an intermediate between two crystalline phases," Shen explained.

Explore further: 'Liquid-liquid' phase transition: Researchers identify transformation in low-temperature water

More information: Chuanlong Lin et al. Experimental evidence of low-density liquid water upon rapid decompression, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1716310115

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Bart_A
not rated yet Feb 13, 2018
Water makes up 70% of our planet? That's news to me! Does someone proofread these articles before they are published here?
TheGhostofOtto1923
5 / 5 (1) Feb 14, 2018
"About 71 percent of the Earth's surface is water-covered"

- If you Google the offending sentence you find that this is just another press release copied in many news feeds. Physorg does not edit or.

You shouldn't be so critical Bart, physorg does not ban religionist fanatics like yourself.

Respect.
TheGhostofOtto1923
5 / 5 (1) Feb 14, 2018
Physorg does not edit PR.

Spellcheck strikes again.
mackita
not rated yet Feb 14, 2018
The fact, that water expands during freezing by 9% of volume is result of high intrinsic compression of water molecules by their mutual extramolecular hydrogen bonds. This intrinsic pressure is about 29.000 bars and it contributes to high phase transition enthalphy of liquid water and wide range of its anomalies. As the result, the water behaves like the liquid gel mixture of oligomeric water clusters and less compressed phase between them. The above study apparently struggles to isolate the later one.
mackita
not rated yet Feb 14, 2018
Rapid decompression key to making low-density liquid water
The above study indicates, that the speed of exchange between water inside the less and more dense phase isn't infinite. It also corresponds previous observations of so-called "autothixotropy" of water (1,2), which means that the water standing at rest is behaving like thin jelly with very fast decaying viscosity after stirring. After some time of standing at rest the original higher viscosity is restored in similar way, like for ketchup standing at rest.
mackita
not rated yet Feb 14, 2018
Note that crunching and squeaking of freshly fallen snow under feet can be also interpreted like the fast relaxation of less dense phase of water, because this fraction covers the surface of ice crystals. Once they move against each other, then the less viscous phase temporarily decreases the friction of crystal surface and after then it becomes viscous again. This leads into famous crunching sound and fast movement of ice crystals, once they get compressed in certain range of temperatures. For temperatures lower than 42 °C the surface phase of ice disappears and the snow behaves like any other dusty matter after then.
nrauhauser
not rated yet Feb 14, 2018
I'm just glad there isn't any Ice IX anywhere on that graph.

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