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Monday, February 11, 2008

Water is weird

The following is a column by Abram Katz, Register Science Editor



Snow has been falling all night, and the air is still below freezing when the neighborhood kids organize a snowball fight.
They compress a handful or two of snow and it sticks together. Why? It’s impossible to make a “sand ball” or a “sand man,” after all.
Intuition tells you that the snowball retains its shape because the water crystals are freezing. Are they melting? No, the air is below the melting point.
Snowballs, thunderstorms, ice skating, frost heaves and bad-tasting drinks all spring from a very strange property of water that scientists are still investigating.
John S. Wettlaufer, professor of geophysics and physics at Yale University, and his colleagues are among a small group of researchers around the world trying to understand water.
We already knew that water is weird stuff — it’s the only common substance whose solid form floats on its liquid form. Everything else works in reverse. Heave a chunk of steel into a cauldron of molten steel and it sinks right to the bottom.
But ice has much stranger properties. In fact, all crystalline materials bear the same trait, Wettlaufer said.
Even at temperatures way below the freezing point, a very thin film of water, or quasi-water, forms on the surface of ice crystals. Free from the force of chemical bonds below, the surface molecules become disorganized and shift around.
This surface melting is what holds snowballs together.
The microscopic properties of freezing and ice surfaces also explains thunderstorms and a host of other large-scale phenomena, from ozone layer damage to potential life on other planets.
Wettlaufer started with the seemingly simple question that has puzzled scientists for hundreds of years: How do things melt?
To understand melting, it helps to start with one- and two-dimensional thought experiments, he said. Rudolf Peierls, a German physicist who died in 1995, did just that.
Imagine an infinitely long chain of atoms connected by springs. “Peierls showed that if melting is defined as ‘loss of long-range order’ then there is no melting point in one dimension,” Wettlaufer said.
Now consider a two-dimensional sheet of connected atoms. There still is no way to create a loss of positional order in the atoms, so there would be no melting point in a flat world.
However, in the 1960s, American mathematician David Murman came up with “topological disorder,” which is different from positional disorder. Think of a loosely woven sheet of cloth, in which the intersection of strands represent molecules.
You could push and pull on the cloth, distorting it. When the cloth finally tears, that’s akin to melting, Wettlaufer said.
What’s interesting is that mathematically, melting in two dimensions is 60 percent easier than in normal three dimensions, because there are fewer interactions, he said.
Practically, it is possible to create a sheet of atoms sitting on a substrate. The sheet is neither two nor three dimensional, but somewhere in between, Wettlaufer said.
Finally, consider a universe full of ice. Slice it, and throw away half. What’s left is an interface between solid ice and water vapor. For quantum mechanical reasons, the atoms on the surface start to move around in a manner that is analogous to the distorted piece of cloth.
Just like the fabric, the surface molecules eventually break loose, forming a liquid. This happens even if the temperature is 50 below. The same liquid layer forms on frozen argon gas.
Water is perfect for studying phase change from solid to liquid to vapor, because it is ubiquitous and occurs in all three forms in an ordinary temperature range, Wettlaufer said.
One source of water is thunderstorms, during which rain falls from clouds that are positively charged at the top, and negatively charged nearer the ground.
Meteorologists knew that colliding ice particles create the charge, but how?
Using two very sensitive scales, an acoustic speaker and two small pieces of ice, one of Wettlaufer’s graduate students measured what happens when two ice particles collide.
An extremely small charge contained in the surface water moves from one particle to the other. The rising particles thus leave the collisions with a positive charge. The movement of positive charges leaves the lower clouds with a growing negative charge. At a critical voltage, the cloud and ground balance the negative charge through a bolt of lightning.
The high-altitude ozone layer, which protects Earth from powerful ultraviolet light, is destroyed by chlorofluorocarbons and other similar chemicals. The reactions that release destructive chlorine compounds occurs in the surface liquid of ice crystals.
This surface water is also responsible for glacier movement and bumps of dirt that rise from frozen ground. The impurities that make drinks taste bad gather in the surface melt. Also, surface melt, rather than pressure melting, explains ice skating, Wettlaufer said.
Extraterrestrial life might form in this liquid on distant frozen moons in the solar system.
And that’s just scratching the surface.
Abram Katz can be reached at akatz@nhregister.com or 789-5719.

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