{"id":4909,"date":"2019-12-19T11:25:59","date_gmt":"2019-12-19T02:25:59","guid":{"rendered":"http:\/\/163.180.4.222\/lab\/?p=4909"},"modified":"2019-12-19T11:25:59","modified_gmt":"2019-12-19T02:25:59","slug":"the-physics-of-ice-skating","status":"publish","type":"post","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=4909","title":{"rendered":"The physics of ice skating"},"content":{"rendered":"

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The slipperiness of ice is poorly understood at a microscopic level. Experiments that probe how the surface of ice melts and flows in response to wear help to explain the exceptionally low friction that underpins winter sports.<\/h5>\n

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It is widely thought that ice skating is enabled by the formation of a layer of water on the surface of ice, which lubricates the skate.\u00a0Writing in\u00a0Physical Review X<\/i><\/a>, Canale\u00a0et al.<\/i>1<\/a><\/sup>\u00a0report that, although there is indeed a layer of something, it is not simply water \u2014 the lubricating layer has surprising flow properties that are very different from those of bulk water, and are intermediate between those of liquid water and ice.<\/p>\n

The idea that a thin film of meltwater wets the surface of ice has been accepted since the nineteenth century2<\/a><\/sup>. This layer should be present beneath skates or other objects sliding on ice. Most of the debate about this topic, therefore, deals with the origin of the layer \u2014 is it the result of surface melting, heating associated with friction generated by the skate, or pressure-induced melting? Pressure melting has largely been discounted, because this process is impossible for ice below about \u201320 \u00b0C, whereas skating is possible at such low temperatures. Surface melting and frictional heating also seem unlikely explanations because these phenomena are not specific to solid water ice, yet ice is almost the only material that it is possible to skate on.<\/p>\n

Also unknown are the mechanical properties of the meltwater layer. Whether a liquid lubricates or not is mostly determined by its resistance to being squeezed out from the gap between the two rubbing surfaces \u2014 thick grease is usually needed for good lubrication. Water is not a good lubricant, because its low viscosity means that it is easily squeezed out of gaps. The idea that a layer of water is sufficient to lubricate a skate on ice is therefore strange. It doesn\u2019t even make intuitive sense, given that it is impossible to skate on a road or a kitchen floor with a layer of water on it.<\/p>\n

Canale\u00a0et al<\/i>. report clever experiments that investigate the microscopic mechanisms responsible for the slipperiness of ice (Fig. 1). The authors measured the coefficient of friction (a measure of the friction produced at a surface) of ice and the properties of the lubrication layer simultaneously, using an experimental set-up that resembles a large tuning fork. The fork was made to vibrate, so that a millimetre-scale glass bead attached to one of its prongs oscillated across an ice surface. The bead thus functioned as a tiny ice skate, gliding for distances of the order of tens of micrometres across the same region of ice.<\/p>\n

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Figure 1 | Probing the lubricating layer of ice.<\/b>\u00a0Canale\u00a0et al<\/i>.1<\/a><\/sup>\u00a0investigated the thin layer of melt water that lubricates the slipping of objects on ice. The authors attached a millimetre-scale glass bead to a prong of a macroscopic aluminium tuning fork. The fork was made to vibrate, causing the bead to oscillate over the ice surface. An accelerometer on the fork measured the amplitude of the bead\u2019s oscillations parallel and perpendicular to the surface. From these measurements, the friction between the bead and the ice, and the flow properties (rheology) of the lubrication layer, could be calculated. The observed viscosity and elasticity of the layer suggest that it consists of a mixture of water and ice particles.<\/span><\/p>\n<\/figcaption><\/figure>\n

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An accelerometer attached to the same prong of the fork as the bead measured the amplitude of the bead\u2019s oscillations parallel to the surface, and compared them with the amplitude of the driving force. Canale\u00a0et al<\/i>. used the difference in amplitude to calculate the friction force between the bead and the ice. Simultaneously, tiny oscillations of the bead (restricted to just a few tens of nanometres, so as not to influence the friction measurements) perpendicular to the ice surface allowed the authors to probe the properties of the lubrication layer. The authors used a similar procedure to the one used to measure parallel movements to measure the forces acting on the bead perpendicular to the surface. The coefficient of friction between glass and ice could then be calculated from the ratio of the two measured forces.<\/p>\n

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