Firing lasers at supercooled water has given us a first glimpse into a "no-go area" that has never been explored before.
Water is an unusual liquid with
"People have wondered for a long time why water behaves this way, and what will happen if you supercool it down to really low temperatures," says Anders Nilsson at Stanford University in California. One controversial idea is that water starts to transform at about -45 °C, entering a "bulk liquid" state in which two different densities of water can coexist.
"If you had a glass of bulk liquid water, it would separate into two liquids, one with a higher density that will sink to the bottom of the glass, and a lower density one that will float on top," says Nilsson. But oddities of liquid water at very low temperatures have long been impossible to study, because below about -45 °C, ice crystals quickly develop in the drops, clouding the measurements.
Nilsson and his team found a way to take snapshots of liquid water cooled to -46 °C. They shot a jet of liquid through a vacuum, creating water drops that rapidly chilled due to evaporation. Some of the drops remained ice-free for about a millisecond – just long enough for the team to hit them with X-ray laser pulses.
"We are only shooting one laser pulse per droplet, and the X-ray is so intense that the droplet explodes," says Nilsson. When the laser encounters a water drop, its light is scattered, creating patterns that reveal the drop's structure and density. Early results show that water gradually evolves towards a low-density liquid as it cools, and that the change accelerates once it reaches the previously unexplored temperature range.
Pablo Debenedetti at Princeton University says the new technique is clever and elegant, adding that it will pave the way for further studies of supercooled water. In addition to uncovering new water physics, fresh insights could have key practical applications.
"Large inventories of supercooled water exist as tiny droplets in high-altitude clouds," says Debenedetti. "The properties of supercooled water are therefore not only of inherent scientific interest, but are also very important in atmospheric science."
Journal reference: Nature, DOI: 10.1038/nature13266
Clarification, 2 July 2014: When this article was first published, we did not make clear that water decreases in density only below 4 °C
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