Freezing eruption. A) Amorphous solid water below 140 K. B) Above 140 K, tiny crystalline grains of ice begin to form. C) Carbon tetrachloride molecules, trapped under the amorphous water layer, percolate up between the ice grains and emerge as a "molecular volcano."

Molecular Eruptions in Water's Weird Cousin

The crystalline ice of snowflakes and winter ponds is a rare commodity in the universe. In the cold reaches of interstellar space, most water probably exists in a strange frozen state, called amorphous solid water. As an upcoming report in Physical Review Letters describes, the behavior of this material is bizarre even when it is changing into ordinary ice: It suddenly fractures into tiny crystals, allowing underlying material to erupt, much like a terrestrial volcanic eruption.

In a regular ice cube, water molecules line up precisely and form a solid crystal. The molecules in amorphous solid water have no such regular structure, but are randomly oriented, like a snapshot of liquid water. To make amorphous solid water, you need water vapor and a surface colder than about 140 kelvins--so cold that the molecules lack the energy to arrange themselves in a crystal lattice after sticking to the surface. When the surface warms up, the amorphous solid water begins to flow, albeit at a glacial pace. This unusual behavior offers a view of water that is helping researchers understand its fundamental nature.

Chemical physicist Bruce Kay and his team at the Pacific Northwest National Laboratory in Richland, Washington, wanted to observe how amorphous solid water interacts with "oily" liquids, which normally avoid water. They spread an ultrathin layer of one such liquid, carbon tetrachloride (CCl4) on a cold metal surface and then added a layer of amorphous solid water on top. As they warmed the sample, the CCl4 suddenly exploded through the water layer.

Curiously, the explosion happened at the same temperature where tiny patches of amorphous solid water begin to crystallize into normal ice. The group suspects that as the water molecules organized themselves into ice crystals, tiny branching cracks opened up, allowing the CCl4 to burst out, similar to lava bursting through cracks in Earth's crust. The CCl4 rushed to escape, because it would normally evaporate at much lower temperatures.

"Studying this amorphous solid water will be useful in terms of understanding the physics and chemistry of liquids," says Kay. The "molecular volcano," he suspects, is just the first of many surprises. Adds Eugene Stanley of Boston University: "It's really quite a dramatic result."

Posted in Physics, Chemistry