When asked to predict the outcome of a competition, coach Carlo Fassi is said to have once replied that one could never be certain what might happen because "the ice is very slippery." And indeed it is. And were it not for this, there would be no skating and no skiing, or any of the other winter sports for that mater.
But WHY is the ice very slippery?
If you ever asked a coach this, you might have been told it is because the pressure of the blade on the ice causes the ice under the blade to melt and this layer of liquid water makes the surface slippery and allows the blade to glide over the ice. A 100 pound skater gliding on one edge, for example, can produce a pressure of several tons per square inch on the ice, and when a solid is compressed under pressure a small amount of energy is absorbed by the solid. In the case of water ice, this energy can cause a small amount of melting.
But not much. Research into the properties of ice over the past few years says that even at several tons per square inch there is not enough melting to account for the slipperiness of ice.
Another hypothesis has been that the motion of the blade over the ice causes frictional heating that melts the surface of the ice under the blade. But this too is probably not the answer.
It turns out ice is slippery for a completely different reason. One that was postulated in the 19th century, and long forgotten about. Experiments on the properties of ice over the past few years suggest this long neglected idea is the likely origin of the low friction of a blade gliding over ice.
Water ice, it turns out, is extremely complicated stuff.
Water is a combination of Hydrogen and Oxygen combined into molecules. In each water molecule there are two Hydrogen atoms for every Oxygen atom. Water can exist as a solid (ice), liquid (what you drink) or a gas (steam). Which form you get depends on the temperature and the pressure of the water. As a liquid or a gas, water flows easily and has very low friction. It turns out, lucky for skaters, that the surface of solid water can also have very low friction.
At normal atmospheric pressure (what we experience at the surface of the earth) water freezes at a temperature of 32 degrees Fahrenheit (0 degrees centigrade, 273 absolute). This is the "bulk" freezing point (or melting if you prefer) of water. When ice cubes are melting in a glass of liquid water the temperature of both the ice and the liquid is 32 F. Energy absorbed by the mixture goes into melting the ice cubes while the temperature stays constant at 32 F.
At normal atmospheric pressure when liquid water freezes it forms a solid crystal with a hexagonal (six sided) microscopic structure. This hexagonal structure is revealed to us in snow flakes which take on the shape of hexagons and six pointed stars. Ice in a skating rink is usually kept at a temperature of about 4 to 8 degrees Fahrenheit below freezing. (Hockey ice is generally maintained colder than figure skating ice.) The layer of ice in a skating rink is a huge crystal of solid water with a hexagonal structure.
Image yourself now as an water molecule buried deep inside the ice. You are completely surrounded by other water molecules in all three directions. You are locked in place. You can't move up or down, or left or right, or in or out. You can't rotate. You can't do anything but vibrate away, shivering where you are.
But what if you were located at the very top layer of the ice?
If you are in the top layer of the ice, you have company in the frozen molecules underneath you, and to the sides, but not on top. So you are not completely trapped. You are actually able to move around some. Not much, but some; and a lot more than when you were buried in the middle. Because of this you are not quite a liquid, but you are also not rigidly a solid.
This top layer of the ice is know as a quasi-liquid-layer. Ice can be pretty cold and still have a quasi-liquid layer, with the thickness of the layer depending on the temperature. Experiments show that this layer exists in ice at temperatures up to about 18 degrees Fahrenheit (10 C) below the freezing point of water. As you warm up, and get closer to the freezing point of water, the quasi-liquid-layer gets thicker, and finally at 32 F (0 C), the surface can begin to melt and a true liquid layer can form. Below 14 F (-10 C) there is no quasi-liquid-layer and ice isn't particular slippery. At very cold temperatures the friction increases tremendously at which point skating on ice would be about as productive as trying to skate on sandpaper! It would be like trying to do cross country skiing at the beach, or pulling a dog sled through the desert. It wouldn't be much fun at all!
In a skating rink, the temperature is kept in the range where a nice slippery quasi-liquid-layer exists. The surface molecules are free to move around some and they are easily pushed out of the way as the blades move over the ice. Because they are free to move around they do not obstruct the motion of the blade or grab onto the blade and slow it down to any significant extent. Ice is slippery because at the temperatures inside ice rinks (and outside in most winter climates) the surface layer of the ice behave like a layer of ball bearings on top of which your blades glide.
So, it's not about pressure, it's not about heating due to friction. Ice is slippery because at the right temperatures its surface just is, and the skater doesn't have to do anything to make it that way.
Copyright 2005 by George S. Rossano
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