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Liquid Water in an Icy No Man’s Land

Scientists probe the strange physics of water at superlow temperatures
supercooled water droplets


Illustration of supercooled water droplets mid-flight through a vacuum chamber, where they are bombarded with intense x-ray pulses. The x-rays scatter as they pass through the water, giving valuable information about its molecular structure.


Credit: Greg Stewart/SLAC

By zapping tiny water droplets with x-ray laser pulses, scientists have gotten their first glimpse into the behavior of supercooled water in a hard-to-reach “no man’s land” of temperatures below –41 degrees C. Understanding water below its normal freezing point of 0 degrees C has been a challenge because it must be handled with extreme care to keep it in liquid form. The resulting insights may help settle a debate among physicists over water’s fundamental properties, including whether it can take on a fourth state beyond the standard three of solid, liquid and gas.
 
Water has strange properties unlike those of almost any other liquid, such as expanding rather than contracting upon freezing, and it gets more bizarre as it gets colder. In fact, water’s oddities start at the warmer temperatures suitable for most of life on Earth, says Stanford University photon scientist Anders Nilsson, senior author of a new paper describing the weird water. “We wanted to go into the supercooling,” he says, “because that’s where everything is amplified in this very strange behavior, and we need to understand where this strange behavior comes from.”
 
Physicists have created supercooled water before, but never this cold and never for long enough to study it closely. To make that leap, Nilsson’s team had to move very fast. Using Stanford’s SLAC National Accelerator Laboratory, they fired tiny water droplets, each the width of a human hair, into a vacuum chamber where they started to evaporate and cool off at a rate of about 100,000 degrees C per second. The team then blasted these supercooled droplets in midair with bursts from an x-ray laser. As the x-rays passed through the water, they scattered, painting a detailed picture of the water’s molecular structure. The whole process took only a few milliseconds per drop, but that was long enough for Nilsson’s team to observe them before they’d hardened into ice. The experiment is detailed in the June 19 issue of Nature (Scientific American is part of Nature Publishing Group).
 
This ultrafast x-ray technique represents a breakthrough. “I thought it was wonderful,” says Pablo G. Debenedetti, a professor at Princeton University who studies supercooled water and is not affiliated with the project. “They were able to extend the range of temperatures over which you can study liquid water.” Previous experiments had been able to study liquid water down to –41 degrees C, but the new study pushed that limit down to –46. The hope is that extending this limit will help resolve a debate about how water behaves when it gets so cold.
 
The debate is over the existence of a “critical point”: a specific temperature below which water would have an additional phase of matter beyond the usual three (solid, liquid and gas). Below the critical temperature, so the theory goes, water would have two distinct liquid states of matter, each with different physical properties such as density and compressibility. Depending on the ambient pressure, supercooled water could be in either one of these states.
 
Up to now, sophisticated computer models offered the only way to study water below –41 degrees C, but they were not accurate enough to predict exactly how real water behaves. Some of the models predict the existence of a critical point whereas others do not. The new technique has yet to settle the debate, but it should give researchers the experimental firepower they need to prove once and for all whether the critical point exists.
 
The answer will not just teach us about supercooled water. If the critical point does exist, it will explain a lot about how water behaves under more ordinary conditions. Although water at room temperature and pressure exists as a single liquid state, little clusters of molecules could coalesce into temporary structures that act like the two supercooled liquid states. “The water doesn’t really know what it wants to be. It’s sort of dancing around locally in small regions of either of these two,” Nilsson says, “and that is why water behaves so weirdly, according to the theory.”

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