Scientists probe the limits of ice | 5/21/2015 | Staff
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How small is the smallest possible particle of ice? It's not a snowflake, measuring at a whopping fraction of an inch. According to new research published in Proceedings of the National Academy of Sciences, the smallest nanodroplet of water in which ice can form is only as big as 90 water molecules—a tenth the size of the smallest virus. At those small scales, according to University of Utah chemistry professor and study co-author Valeria Molinero, the transition between ice and water gets a little frizzy.

"When you have a glass of water with ice, you do not see the water in the glass turn all ice and all liquid as a function of time," she says. In the smallest water nanodroplets, she says, that's exactly what happens.

Transition - Water - Ice - Transformations - Phases

The transition between water and ice is among the most important transformations between phases (solids, liquids and gases) on our planet, where it has unique effects on our climate while also regulating the viability of life. Understanding the conditions that lead to the formation of ice, then, is an active quest in areas that encompass environmental and earth sciences, physics, chemistry, biology and engineering.

Ice exists on Earth almost exclusively in the highly ordered hexagonal crystal structure known as "ice I." In our atmosphere, small water clusters form and subsequently freeze, seeding larger crystals and eventually clouds. Due to competing thermodynamic effects, however, below a certain diameter these water clusters cannot form thermodynamically stable ice I. The exact size range of water clusters capable of forming stable ice I has been investigated through experiment and theory for years with most recent estimates narrowing the range from as low as 90 water molecules to as high as 400.

Past - Barrier - Limit - Liquid - Clusters

In the past, a major barrier in experimentally studying this limit has been cooling the supercooled liquid clusters slow enough...
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