ROBERT C. PLUMB
chemical principles exemplified
W~mesIerPolytechnic Imlllule Worcea... M O I S ~ L ~ Y M 01609 IIS
Sliding Friction and Skiing Illustmfing principles of surface chemistry
Suggestion and information provided by Professor Alexander V a n Hook, University of Tennessee It can be exhilarating t o escape the constraints of friction; the parachutist's ultimate thrill is free falling in space, but most of us settle for less-for example, running and sliding on patches of ice, skating, or sliding down a mountain on skis. I t is well known that the low friction of skiing results from a thin film of liquid water between the snow and the ski. The interesting scientific question is, what causes the film of water t o be there? Several possiblc answers, and the experiments which prove or disprove them, illustrate a variety of principles of surface chemistry. Many of the unusual properties of snow and ice result from thc fact that the molar volume of ice is greater than that of liquid water; so that ice, near O°C, melts when pressure is applied t o it.l One might suspect that the film of water between a ski and snow- is produced by pressure melt,ing. If that were true then the frictional behavior of other solids near their melting points would be different from that of ice, because most other solids have a molar volume which is less than that of the liquid. The fact is that the coefficients of friction of sliders on krypton and carbon dioxide, somewhat below their melting points, are as low as on ice and vary in a corresponding manner with temperature. Pressure melting of krypton and carbon dioxide do not occur, yct the friction measurements suggest that you could ski as readily on solid krypton (mp -15G.G°C) or carbon dioxide (-56.6'C a t 5.2 atm) as on ice. Although some pressure melting undoubtedly occurs during skiing, some more general principle must be operative. The molecular structure of ice a t a surface must be different from in the bulk of a crystal. This is necessary on thermodynamic grounds. However, the structure of the surface layer and its thickness are not known. It has been suggested that the surface layer is "liquidlike" and that the thickness of the "liquid-like" layer is a maximum just below the melting point and drops t o essentially zero thickness a t -20°C. So perhaps snow is slippery because it is coated with liquid water. This model would explain why the friction of a ski on snow becomes greater as the temperature drops. Of course one would have to also conclude from the fric'tional behavior of frozen krypton and carbon dioxide that these solids and perhaps all solids have a liquid-like surface 830
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Journal of Chemical Education
structure near their melting points. Even if the surface layer of ice is liquid-like, that does not explain thc origin of the water film during skiing. Bowden and co-workers imbedded two electrodes in a ski and measured the conductivity as it slid over salty ice. They showed that the water film between a ski and snow is continuous during sliding if the temperature is close t o the melting point, but little if any film is present when the ski is stationary. Thus the water film is produced by the sliding. The generally accepted origin of the water film is that the thermal energy released by friction between the snow crystals and ski causes melting and produces a thin layer of water. The shear of t,his water film as the ski moves provides the thermal energy required t o maintain it as a liquid against the opposing tendency of thermal conduction which would cause it t o refreeze. Calculations of the rate of viscous dissipation of thermal energy and heat losses by conduction support the model. Skis having surfaces of differing thermal conductivity vary in performance as expected. A brass ski, having good thermal conductivity, has high friction because the frictional heat is carried away quickly and the liquid film cannot be maintained. An ebonite ski, on the other hand, slides better because ebonite is an insulator. Arctic explorers in the pre-snowmobile days recognized this effect. One of them compared two sledges-one with nickel plated metal runners and the other with maple runners-and reported ". . . it was a t least half as hard again t o draw a sledge on the nickel runners as on the tarred maple runners." As far as the coefficient of friction is concerned, it 'doesn't really matter greatly what the skis are made of or coated with if the snow is just a few degrees below the melting point. However, as the temperature drops lower, a continuous lubricating liquid film is not maintained. The problem is then more complicated, that of a solid sliding on a solid powder with only limited melting taking place a t limited areas of contact. With modern ski materials and waxes we can have fairly fast skiing even without the ~ a t e r ~ f i l m . Suggestions for Further Reading BOWDEN, F. P., and T ~ o nD., , "The Friction and Lubrication of Solids," Clarendon Press. Oxford. 1950. o. 66-71. ADAMBON, A. lV.. "Physiaal che.mistly of S U T ~ & DIntemeienoe, ~S." New York, 1960. p. 335-7. J E L L I N E H H. H. G . . in "Water and Aqueous Solutions: Structure, Thermodynamios and Transport Pro~erties." (Ediioi: Honrr;, R. A.) VilevInterscience. Nerv York. 1972, p. 66-107.
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See this column, March 1972.
