If you should go skating: The slippery truth about ice

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  • Published: Dec 15, 2015
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: If you should go skating: The slippery truth about ice

Skate on

An icy winter's morning has the scientifically minded wondering about the deceived wisdom of slipping and sliding on ice. Now, a new equation for the frictional shear stress of ice could lead to better ski and skate designs and help us understand better the movement of glaciers and perhaps even the failing of icesheets.

An icy winter's morning has the scientifically minded wondering about the deceived wisdom of slipping and sliding on ice. Now, a new equation for the frictional shear stress of ice could lead to better ski designs and help us understand better the movement of glaciers and perhaps even the failing of ice sheets.

Many authors have discussed what it is that makes ice slippery and why skaters and skiers alike can glide with such seeming effortlessness. Is it pressure effects lowering the melting point and leading to a wet outer layer, frictional heating or something else? Bo Persson of the Jülich Research Center in Germany wanted to freeze the action and has now devised a theory that describes the degree of slipperiness achieved on ice when a hard material such as a ski slides across it. Persson's equation for the behaviour of ice seems to agree well with the experimental data and could help in the design of better skis. It might also contribute to a clearer understanding of ice friction and the fundamentals of glacier motion and other natural processes, such as collapsing ice sheets. Details are reported in the Journal of Chemical Physics.

Pre-melting

Ice, it seems, is not actually slippery, it only really becomes so should a thin layer of liquid water form on the surface, but that layer need not be more than a few molecules deep to cause all kinds of problems for walkers, drivers and others but also add to the fun of so-called winter sports that rely on a friction free zone for skating and skiing. The water can appear when heat from friction melts the ice, or via a natural solid to liquid phase transition, known as pre-melting. Pre-melting occurs near the surface even at temperatures well below the bulk freezing temperature of the water and is a phenomenon that was known to that great nineteenth century polymath Michael Faraday 150 years ago. Faraday observed that two touching ice cubes would adhere to one another and determined that there must be a liquid-like (a pre-molten) layer on the surface of the ice that re-freezes when the two cubes come into contact.

One aspect of the question of slippery ice that complicates matters still further is that the study of ice friction between ice and a sliding object occurs at the interface between two solids, which is called a buried interface. "It is nearly impossible to study directly at the molecular level what happens at the buried interface," Persson laments. This is because the solid material blocks the contact area and you cannot scatter particles like ions or electrons from the interface to study it, as you might for a free surface. Although pre-melting of the top layer of a free ice surface has been studied extensively both theoretically and experimentally, the extent to which the results apply to the buried contact area between ice and another solid materials is not at all clear, Persson says.

Theoretical description

There is, however, a wide range of experimental data that shows how temperature and sliding speed affect friction on ice. Persson's main breakthrough has been to relate a theoretical description of ice friction to this experimental data and he did so by developing an equation that describes the frictional shear stress in ice. This is the force experienced by the material at the area of real contact from a force parallel to the surface, such as that induced by the sliding of a ski across the ice. The equation derived by Persson reveals how the shear stress depends on the ice temperature at the surface. It also suggests that, as with a free surface, the buried ice interfaces may also exhibit pre-melting behaviour.

"The most important result is that I have constructed a phenomenological shear stress law which is able to explain ice friction as a function of sliding speed and temperature in a wide velocity and temperature region," Persson explains. Persson's primary interest is in explaining the physical origin of friction on ice and he has already extended the equation to help explain how rubber behaves on ice, which has obvious implications for vehicle safety in icy conditions, given that rubber tyres can deform.

Related Links

J Chem Phys 2015, 143, 224701: "Ice friction: Role of non-uniform frictional heating and ice premelting"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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