Why ice is slippery
R. David Whitby | TLT Worldwide July 2021
Researchers look into a major scientific challenge that remains difficult to address.
Everyone knows that ice is slippery. Exactly why that is so has eluded tribologists, physicists and chemists for almost 150 years.
It has been thought that ice is slippery due to the presence of a layer of water on the surface of ice and that this acts as a lubricant. The surface layer that allows ice skating has been ascribed to either pressure-induced or friction-induced melting of the ice surface or to the existence of a pre-melted layer of ice. Some research suggests that the diffusion of water molecules over the surface of the ice provides low friction at high temperatures and slow sliding speeds. Reciprocating ball-on-ice friction experiments using a tuning fork are believed to indicate that a mixture of liquid water and ice particles dominates the frictional behavior. For all of these proposed lubrication mechanisms, a crucial parameter that remains ill understood is the local contact pressure exerted at the slider-on-ice interface.
Now, Rinse Liefferink and his colleagues at the University of Amsterdam and the Advanced Research Centre for Nanolithography in Amsterdam have published
1 the results of slider-on-ice friction experiments as a function of temperature, contact pressure and speed. The experimental methods involved dragging various sliders over an ice surface at varying speeds, while continuously tracking the forces on the object and the ice temperature. The sliders were made from a dense plastic with properties similar to ice and, in some experiments, water was dripped over the surface to simulate a water layer.
The frictional properties of ice, which are particularly important for winter sports, the movement of glaciers and the behavior of vehicles on icy roads, are generated by an interface that includes many discrete contact points due to the surface irregularities on the ice and the slider. Understanding how this extended interface impacts the slipperiness of ice is a major scientific challenge that remains difficult to address, because the interface is sandwiched between two bulk materials. The experiments have overcome this challenge and discovered that ice friction is low because of mobile ice molecules at the surface, whose mobility, and therefore slipperiness, can be suppressed by high contact pressures or low temperatures.
The experiments found that friction was at a minimum at temperatures around -10 C. At higher temperatures, the ice surface is plastically deformed due to the pressure exerted by the slider, a process depending on the slider geometry and penetration hardness of the ice. This slows the slider’s movement. At lower temperatures, the friction between the surface and the sliders increased dramatically, again slowing the slider’s movement. At temperatures well below the melting point, friction is strongly temperature dependent and follows an Arrhenius behavior, which the researchers interpret as resulting from the thermally activated diffusive motion of surface ice molecules. They have observed that this motion is hindered when the contact pressure is increased. In this case, the friction increases exponentially, and the slipperiness of the ice disappears. The ice penetration hardness was found to increase approximately linearly with decreasing temperature and sub-linearly with indentation speed. The experiments also showed that the sliders only glided easily after reaching a speed of about 1 meter per second when water was being dripped onto the surface. If there was a water layer on the ice, its effect is only interesting when the sliding speed is higher than 1 meter per second, and the pressure put on the surface is above a certain value. However, as they explain, children are not usually in the higher speed regime and are generally not sufficiently heavy but are still able to skate on ice, so the layer of water is not the complete answer. The results suggest that the slipperiness of ice is governed mainly by water molecules bouncing around on the topmost layer of the ice.
They have concluded that it is the high mobility of the water molecules in the outermost layer of the ice, combined with the exceptional hardness of ice close to its melting point, that make it slippery. Ice friction can be minimized by curtailing the contact pressure, a factor already controlled by ice skaters: The optimal ice skate has a smooth bottom (low pressure) for low friction and sharp edges (high pressure) for grip.
REFERENCE
1.
Liefferink, R.W., Hsia, F.-C., Weber, B. and Bonn, D. (2021). “Friction on ice: How temperature, pressure, and speed control the slipperiness of ice,”
Phys. Rev. X, 11, (1) 011025.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.