Watch your step! This coot is standing on a semi-liquid interface between ice and air, so it had better tread lightly.
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Chemists and physicists have heavily debated the seemingly simple question of why ice is slippery. Intuition tells us that liquids are mobile and that their presence reduces friction between solids, which is why water on the floor can cause someone to slip. Yet ice is frozen water – a solid. Therefore, in order to make that solid slippery a liquid must form that allows skates to slip. But how is that thin layer of liquid water going to appear if ice’s temperature is well under melting temperature?
A common conception is that pressure melting creates this thin layer of liquid water. Pressure melting is the idea the person or skater’s weight exerts an exorbitant amount of pressure per square inch of ice that causes ice to drop its melting temperature. In addition, ice is less dense than water. The lower density of ice means that the melting temperature of ice can be lowered below the usual 0°C by squeezing it (2). Increasing pressure reduces the volume available; therefore Le Chatelier’s Principle will favor the melting of water to compensate the loss of volume due to pressure(1).
James Thomason proposed these ideas in 1850 when he calculated that a pressure of 466 atmospheres would correspond to a melting pressure of -3.5°C(1). However, he was not able to explain how hockey players and figure skaters were able to slide at temperatures below -3.5°C. It is well known that skating is possible at very cold temperatures from around -30°C, so how is it possible for skaters to skate at this very cold temperature? Their own weight would not be able to pressure the ice enough to drop the melting temperature of ice and create a thin layer of liquid water.
Frank P. Bowden and T.P. Hughes suggested the frictional heating alternative to the pressure melting theory in a 1939 article (3). Bowden and Hughes did an experiment at a research station in Switzerland where temperatures did not rise above -3°C. The team achieved still lower temperatures by using solid CO2 and liquid air. Using surfaces of wood and metal, they measured the effects of static and kinetic friction on ice melt. The researchers concluded that frictional heating was responsible for melting ice. Although frictional heating may answer why ice is slippery when moving, this theory does not explain why ice can be so slippery even for someone standing still on it.
In 1850 Michael Faraday suggested that a film of water on ice would freeze when placed between two pieces of ice, but that the film would remain liquid on the surface of a single piece (1). His experiments were the first to investigate the phenomenon of pre-melting, the development of a liquid layer that forms on solids at temperatures below the melting point. Unfortunately, he was not able to reason at the molecular level why this occurred. It was not until 1949 that C. Gurney suggested that an intrinsic liquid film plays a role in the slipperiness of ice. Gurney hypothesized that molecules, inherently unstable at the surface due to the lack of molecules above them, migrate into the solid until the surface becomes unstable, which prompts the formation of a liquid phase.
One of the most convincing experiments that supported this theory of a film of water around ice was conducted in 2004. Harald Reichert studied the interface between ice and solid silicon dioxide using x-ray reflectivity and calculated the thickness and density of the liquid layer between -25°C and 0°C. The density of the surface phase varied from that of liquid water at its melting point to 1.16 g/cm3 at -17°C, like a “high-density form of amorphous ice” (1). This experiment proved the existence of a liquid-like layer around ice that is probably the main cause of ice’s slipperiness.
Theorists have tried to clarify the nature of the liquid-like layer. In 2004 Katsuyuki Kawamura performed experiments that led him conclude that this thin liquid-like layer is formed due to the reduced number of chemical bonds holding the surface molecules in place (4). Atoms in the outermost surface vibrate with greater amplitude than atoms in the interior solid. Surface melting is attributable to the interaction of the vibrational motion of the surface molecules with the interior of bulk molecules.
Surface melting and the creation of a liquid-like layer on ice gives ice its slipperiness. However, it is also important to realize the importance of pressure melting and frictional heating on ice. Both of these processes can develop true liquid layers on ice. Overall, these discoveries represent a huge breakthrough in chemistry and physics because they explain the nature of surface molecules in solids – knowledge that can be applied and studied in different solids, possibly even to make cement slippery.
References:
1. Rosenberg, Robert. “Why Is Ice Slippery?” Physics Today (December 2005): Web. 6 Apr. 2013.
2. Chang, Kenneth. “Explaining Ice: The Answers Are Slippery.” New York Times. N.p., 21 Feb. 2006. Web. 6 Apr. 2013.
3. F.P. Bowden, T. P. Hughes, Proc. R. Soc. London A172, 280 (1939). See also F.P. Bowden, Proc. R. Soc. London A217, 462 (1953)
4. T.T. Ikeda-Fukazawa, K.K. Kawamura, J. Chem. Phys. 120, 1395-401 (2004).