Ever wondered what makes ice so special to skate on it? Apart from freezing your favorite beverages, it helps make life adventurous. Yes, I am talking about sports like ice skating and ice hockey, it’s amusing when you think that there are national level sports leagues built around our beloved ice. Well, years of research finally answered our queries. Grab an ‘iced’ tea or coffee before we start our journey.
Recently a Dutch team (yes, Dutch are crazy about ice skating!) from the Institute of Physics, University of Amsterdam and Advanced Research Center for Nanolithography presented a detailed research on the friction developed on ice. Countless number of experiments and studies brought them to the conclusion that friction on ice majorly depends on parameters like temperature, ploughing, local contact pressure, sliding speed and substrate. Now, let’s try to understand the relationship of ice with the above parameters based on sphere-on-ice friction experiments.
The team performed reciprocating spherical slider experiment in which different kinds of materials (silicon carbide; soda-lime glass; sapphire sphere and stainless steel) for a sphere were used. This was done in order to understand the influence of surface topology w.r.t friction. A rheometer (An electrical device used to understand the way in which a liquid flow’s in response to the applied forces.) is clamped to the slider at a distance of 5mm from the rotation axis. A thermocouple was also embedded near the surface to measure its temperature. The temperature control of this setup was done using cooling liquid.
The main aim of having sliders with different materials was to understand the frictional behavior of the surface under surfaces with different roughness. The surface topology of the of the spheres was studied with the help of a laser-scanning confocal microscope. To get the desired surface roughness, sandpapers of different grades were used.
Now that there is a good understanding about the apparatus used in this experiment, let’s try to understand the mechanical properties of the ice surface. The first property is the hardness of the surface, for which indentation experiments were carried out at various temperatures. Additionally, elastic modulus and poisson’s ratio were also considered.
Temperature plays a key role in determining the coefficient of friction of ice when it’s below the melting point. When an experiment is done by taking three different contact surfaces with a constant sliding speed, it is observed that the frictional coefficient follows Arrhenius temperature dependence. The activation energy obtained is quite close to the activation energy of ice surface diffusion. This is a supporting statement for the idea that diffusion of water over the ice surface plays an important role in ice friction.
This experiment was carried out using three different sliding surfaces, where two of the spheres were made of SiC but with different radius and the other was a model skate. By referring from the above graph we can clearly observe that there is an increase in the co-efficient of friction after the temperature raised from -20 °C. This is because of ploughing friction, as the ice surface acts as a plastic at this temperature and the slider ploughs through the surface of ice increasing the friction.
The phenomenon of ploughing generally occurs when the contact pressure is greater than the penetration hardness of the surface. It is important to note that the penetration hardness decreases linearly w.r.t increase in the temperature.
We can come to a conclusion that the decrease in the radius of the sphere and the hardness of the material will result in increase of the ploughing force by referring the above graph, obtained from an experiment conducted with spheres of different radiuses at constant sliding speed.
With this knowledge, a better understanding on what happens when we increase or decrease our pace while ice skating can be obtained. While cruising at higher speeds we generally align the blades with the direction of our motion as the blades have larger radius of curvature along their length, it creates less ploughing in turn creating smaller coefficient of friction between the surfaces. But if we want to decrease our pace we tend to tilt our skates resulting in deeper indentation and ploughing.
Local contact pressure
There is a good influence of surface topology of the contact material on the magnitude of local contact pressure. If the roughness peaks on the surface are sharper, this yields a higher contact pressure. Additionally, the mechanical properties of the contact surfaces play a key role in determining the local contact pressure. Temperature does not play a significant role in determining the contact pressure as the change in the elastic modulus of ice w.r.t to temperature is quite negligible.
The relationship between sliding speed and the lubricant is quite interesting. Generally, when the sliding speed increases more lubricant is brought between the contact surfaces which give the lubricant the ability to partially support the load. But in case if higher speeds are reached the fluid in between acts as a film that separates the two solids by increasing the friction.
To understand the effect of water lubrication, an experiment with high-density polyethylene (This material has similar mechanical properties to that of ice surface) was brought into picture with water-lubricated substrate.
With the help of the above figure where the open circles are the results related to the dry contact surfaces, while the closed circles belong to the surfaces with lubrication, a clear understanding of substrate effect on the coefficient of friction can be obtained. The curve for wet substrate shows a behavior similar to the Stribeck curve – a well known Master curve in tribology. At low speeds, a boundary lubrication is observed which is associated with higher friction. In this regime, lubricant film is too thin to separate the contacting surfaces, which results in high friction. With the increase of speed, the lubricant film gets thicker, the “dry” contact area decreases and friction drops. In the above graph, up to speed of 1 m/s, the boundary lubrication is observed, while at higher speed a transition to mixed lubrication. This makes the friction drop fast. This brought the researchers to a conclusion that at least up to speed of 1 m/s, the lubrication with water is not the main mechanism in slipperiness of the ice (even though the remark is made that HDPE and ice may behave differently).
The researchers conclude that ice friction is low not due to water lubrication, but due to high mobility of the water molecules at the slider-ice interface. Long story short, a following advice can be given to ice-skaters and ice hockey players: make your skates smooth and with sharp edges. The smooth surface will ensure low pressure and consequently low friction, while sharp edges will be useful if you would like to stop once in a while by creating high pressure and ploughing.
 Rinse W. Liefferink, Feng-Chun Hsia, Bart Weber and Daniel Bonn, Friction on Ice: How Temperature, Pressure, and Speed Control the Slipperiness of Ice.