Ice is an inescapable fact of life in Canada every winter. It makes roads and sidewalks slippery, bursts pipes, cracks pavements, heaves ground. It can bring down trees and power lines and even airplanes.
And yet, if ice didn’t have the special properties that make it sometimes destructive and almost always a nuisance, two vitally important things might be downright impossible: life on Earth, and hockey.
Ice is, of course, frozen water. When most substances change from liquid to solid, they shrink and become denser. Water is unique in doing the opposite: it expands and become less dense when it freezes.
A water molecule consists of two hydrogen atoms bound to an oxygen atom in a V-shape, with an angle of 105 degrees. When these molecules are finally cooled down enough to lock together into the crystalline latticework of ice, they take up more space than when they’re free to tumble around each other as a liquid.
This is good, because otherwise there would be no life on Earth. If ice were denser than water, it would sink to the bottom of the oceans and lakes, piling up until they were nothing but pits of ice with, at best, a little meltwater on top–no room or warmth in them for life.
But because ice floats, it forms a layer of insulation as it forms. This slows the freezing of the water beneath it, which freezes only as it comes into contact with the ice “lid.” As a result, lakes freeze from the top down, and any sufficiently deep body of water doesn’t freeze all the way to the bottom–to the relief of anything living in it.
The fact that ice floats is what makes life possible on Earth. The fact that it’s slippery is what makes hockey possible.
I wrote a column on skating a few years ago and stated (as was commonly believed at the time) that skating works because pressure from the blade melts the ice, creating a thin layer of water that makes the ice slippery. Alas, somebody finally did some calculations, and discovered that neither skates, pucks, nor boots generate enough pressure to instantly liquefy ice and make a slippery layer of water. Instead, all ice, even ice cooled to -129 Celsius or below, is covered by a permanently slippery “quasi-fluid” layer. In this layer, just a few molecules thick, some of the molecules vibrate rapidly (as they would at a higher temperature), but only up or down, not in all directions as they would in liquid.
This quasi-fluid layer is proving useful: Victor Petrenko, a scientist at Dartmouth College in New Hampshire, has figured out how to use it to clear power lines of ice.
Besides its other peculiar qualities, ice is a “protonic” semiconductor. In an ordinary semiconductor, such as the silicon in a computer chip, current flows as a stream of electrons; but in ice, current flows as a stream of protons (hydrogen atoms whose single electron has been stripped away). This movement of protons can cause the water molecules within the ice’s crystal lattice to rotate. It turns out the quasi-liquid layer that makes ice slippery also conducts electricity 100 times better than solid ice. Sending a current through this layer increases molecular motion so much that the quasi-liquid layer turns into a heating element, melting the ice beneath it.
Petrenko’s system, developed with a consortium of U.S. and Canadian power companies, sends high-frequency electrical signals along power cables to do just that. All it requires are small power units placed every 100 kilometres or so along the lines, producing about 50 watts. It doesn’t reduce electricity transmission and it should cost a fraction of what it normally takes to keep power lines clear.
Petrenko thinks you could also use the same idea to make skis go faster or slower. A steady flow of electricity across electrodes on the bottom of a ski would create a layer of lubrication. On the other hand, a quick pulse of electricity–less than a millisecond–would melt the snow’s surface but refreeze it almost instantly. The re-forming ice crystals would increase friction, acting as a brake. The same principle could be applied to the soles of shoes or boots, and possibly even tires, to increase grip.
Even with global warming, ice isn’t going to vanish from our lives in Canada. But the more we know about it, the better we can cope.
