Supercooling liquid water to temperatures lower than ever achieved before has revealed new evidence that water can exist as two different liquids simultaneously.
Supercooled water 鈥� liquid water cooled below its freezing point without being allowed to freeze 鈥� has been baffling chemists for decades. Previous studies found that the extent to which water molecules pack together, known as their density, starts to fluctuate more and more as water is cooled to extremely low temperatures.
Since then, evidence has been mounting that these fluctuations may indicate the presence of two different liquids in one, with some water molecules packed more closely together and others spaced further apart.
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But studies trying to capture these two different liquid structures and analyse them have been limited, because supercooled water usually freezes nanoseconds after it is formed and the precise temperature range within which water might adopt this bizarre two-liquid state wasn鈥檛 known.
By firing lasers at an extremely thin sheet of ice, Greg Kimmel and Bruce Kay at the Pacific Northwest National Laboratory in Washington were able to briefly generate 鈥� and analyse 鈥� supercooled liquid water at much lower temperatures than has been possible previously.
During the fraction of a second in which the water was in its supercooled state, the team took a snapshot of its structure using infrared spectroscopy, a method that takes advantage of the way infrared light is transmitted through molecules. Repeated experiments revealed fluctuations in the density of the supercooled liquid consistent with the two-liquid hypothesis.
The group was able to identify a temperature range within which supercooled water transitions between its two liquid forms: between about -93掳C and -33掳C. 鈥淭his is an important piece of the puzzle that was not previously available,鈥� the team says.
By increasing or decreasing the temperature within the critical range, the team also discovered that you can tip the ratio of the two liquids one way or the other, suggesting the two-liquid phenomenon can鈥檛 simply be explained by water crystallising into ice. 鈥淲e noticed that there was something funny happening early on before it was really crystallising,鈥� says Kimmel.
鈥淲ater has many strange properties,鈥� says Anders Nilsson at Stockholm University in Sweden. But this is the first time there has been such clear experimental evidence of two liquid structures, he says.
鈥淭he many phenomena connected to water鈥檚 microscopic behaviour are pivotal for our life,鈥� says Paola Gallo at Roma Tre University in Italy. Understanding water鈥檚 unique chemistry could help us predict how it might behave under unusual conditions, such as in outer space, she says.
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