Ultrafast laser records droplet temperature at –46° Celsius
Gregory Stewart, SLAC National Accelerator Laboratory
Using a miniature water gun and a powerful laser, researchers have probed tiny water droplets at –46° Celsius, the lowest temperature that ordinary water has ever been detected in the liquid phase.
“It’s a world record, and it’s hard to imagine it will ever fall,” says H. Eugene Stanley, a physicist at Boston University.
The study, published in the June 19 Nature, marks the first time that scientists have observed liquid water in the low-temperature region where water’s already unusual properties are expected to become even weirder. By studying such cold conditions, researchers hope to understand water’s quirks at all temperatures and how life takes advantage of them.
Water’s odd properties make it special for life. Unlike most substances, water is denser in liquid form than as a solid, which allows aquatic creatures to survive in pockets of liquid beneath the frozen surface of a lake. Water has other properties, including its ability to absorb heat and low compressibility, that change suddenly at very low temperatures. Scientists have been eager to explore what triggers these changes in the extreme cold. “If you understand water there, you understand it everywhere,” says Anders Nilsson, a physicist at Stanford University and the SLAC National Accelerator Laboratory in Menlo Park, Calif.
Though water usually freezes at 0° C, pure H2O can remain liquid well below that temperature. This “supercooled” water looks just like water from the tap, but it is far more delicate: It solidifies the moment it encounters a foreign surface, such as a dust grain, ice cube tray or window of a cruising airliner. Past experiments had detected liquid water at –38° C but not colder, despite scientists’ suspicion that it could subsist for measurable periods of time at even lower temperatures.
To keep water cold and uncontaminated, Nilsson and colleagues squirted small water droplets about the size of red blood cells into a vacuum chamber. As each droplet traveled, some of its component molecules evaporated, releasing heat and causing the droplet’s temperature to plummet as much as 10 degrees per millisecond.
As the supercooled droplets journeyed through the chamber, the researchers used SLAC’s X-ray laser to serve as a flash camera. The laser zapped ultrashort pulses — each 50 millionths of a billionth of a second in duration — that struck some of the droplets. The laser energy caused each unlucky zapped droplet to explode, but not before X-rays had passed through and delivered the equivalent of a medical scan to a detector on the other side. “It’s like a hit-and-run,” Nilsson says.
The results revealed that some of the droplets existed in liquid form at temperatures as low as –46° C and could remain in that state for about a millisecond before freezing. That’s a longer time than some scientists expected, says Princeton University chemical engineer Pablo Debenedetti. “It’s a major experimental accomplishment,” he says.
The new work also shows that water’s weirdness gets more extreme as the temperature drops, Stanley says. That lends support to the idea, proposed by Stanley and colleagues 22 years ago, that supercooled water undergoes a phase change at around –50° C into two never-before-observed liquid states. One of those novel phases should exist only when water is under pressure. Nilsson plans to subject the droplets to a combination of low temperature and high pressure to try to observe the dual phases.
A new computer simulation, described by Debenedetti and colleagues in the same issue of Nature, provides further evidence that water splits into two liquid phases at low temperatures.
J.A. Sellberg et al. Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature. Nature. Vol. 510, June 19, 2014, p. 381. doi: 10.1038/nature13266.