Sprinters are often dubbed "flyers", such as Bolt, who can reach speeds of 37.5 km/h. That's fast enough, but with the help of the ice knives, speed skaters can reach high speeds of more than 50 km/h on the field. In the case of skids, the speed can even reach 140 km / h.
Figureworm creative
To achieve such a high speed, in addition to the hard training of athletes and the efforts of R & D personnel, the most fundamental reason is that ice, it is slippery... At first glance, this is nonsense, but when you dig deeper, you will find that things are not simple, ice, how did it slip?
Ice is so slippery because of water.
We have all studied a little physics and know that the so-called "dry grinding" friction will be greater, such as the shaft on the door is not lubricated for a long time, and it will be laborious and make a harsh sound when opening and closing, but if lubricants are added, such as lubricating oil, the door suddenly becomes quiet and easy. The same is true on ice, there is a film of water between the ice knife and the ice, which plays a lubricating role, so that the resistance of the ice knife is greatly reduced, and it can slide at high speed.
But where does this water come from? This issue is a lot of controversy, and we will talk about it next.
01
Is the water pressed out?
A long-standing theory is that when the surface pressure of ice becomes stronger, the melting point will decrease, and this is how the ice knife "compresses" the ice surface into water. There is also a very classic science experiment can prove this, the two ends of the heavy wire straddling the ice, the ice block touching the lower edge of the wire will melt so that the wire sinks in, and after the wire passes, the melted ice can be frozen into one. Finally the wire will pass through the ice cube, and the ice will not break, this is the principle.
According to this theory, the speed skating ice knife looks very narrow, about 1.4 mm thick, and the actual area in contact with the ice surface is very small, while the short-track speed skating ice knife has a little curvature and the contact ice surface area is smaller. In this way, the pressure of the ice blade on the ice is very large, resulting in a reduced melting point of the contact part of the ice, which can produce a lubricated water layer... Do you?
While it sounds reasonable, and it is true that increased pressure leads to a decrease in the melting point of ice, unfortunately this theory is wrong.
Because, according to the researchers' calculations, a speed skater weighing about 70 kilograms, wearing a skate and standing on the ice, the ice contact area is calculated according to 0.0001 square meters, in this case, it can only reduce the melting point of the pressurized ice by 0.5 ° C. But in fact, the ice surface temperature that is more suitable for speed skating is usually at -5 °C ~ -9 °C, and the change of 0.5 °C is tantamount to a cup of water.
Moreover, we can also find that, on the one hand, powerful speed skaters are often more symmetrical and slender, and do not need the kind of muscle fierce people with large weight to press the ice, on the other hand, even if they are very light children, as long as they learn the action, they can still skate fast, so, combined with theoretical calculations and common sense, the lubricated water when skating is probably not "pressed" out.
02
Is the water rubbed out?
There is also a theory that the sliding of the ice knife will produce friction, and friction can generate heat, and heat can turn ice into water, making the ice surface more slippery.
To test this, as early as 1939, Bowden and Hughes, near the Jungfrau research station in Switzerland at an altitude of 3346 meters, planed an ice cave and experimented with the complex device below. The temperature there is relatively constant, not exceeding -3 °C, and the air pollution is very small, which can reduce the interference with the experiment. The reason why I chose to do experiments here may be because the conditions of that year were limited and it was inconvenient in ordinary laboratories.
Friction device, from references[1]
To briefly summarize the principle of this experiment, it is to freeze out a flat circular ice surface (C in the figure), then put a slider (I in the figure) on the ice surface, and then use the motor to drive the ice surface to rotate and test the friction generated at different temperatures and counterweights. It was found that the lower the temperature, the more difficult it was to form a water film, and the greater the friction, which is why the outdoor ice rink temperature is too low and the ice is more astringent. During long-term experiments, obvious signs of melting can be found on the ice surface at the friction.
There is another sport on the Winter Olympics that can prove this from the side, that is, curling. Ice sweepers use brushes to rub high speeds across the field, generating heat that slightly melts Xiaoice particles on the curling field, allowing curlers to slide farther, fine-tuning the curling route.
Curling pod enlargement from references[2]
There may be some friends who will wonder, so a large brush to brush ice back and forth to produce heat can be imagined, but when speed skating, the ice surface is so large, narrow ice knife rubbing so, can really rub the ice? In fact, from a microscopic point of view, the ice surface and the blade of the ice knife are not smooth, but there are many tiny trenches and bumps, according to Bowden and Hughes estimates, the friction heat in a short period of time, if concentrated on a small point, is enough to make that little bit of ice rise from -20 °C to 0 °C. To lubricate the ice blade and ice surface, only a little water is enough, and as mentioned earlier, the ice surface temperature of the indoor speed skating field will not be lower than -10 °C.
It turns out that ice skating relies on friction to generate heat, and we may come to this conclusion when we see this. But does that really end there? If water is generated only by friction, then why do people sometimes slip when they stand on the ice? This has to do with the nature of the ice itself.
03
Does water come with ice?
We tend to think that ice is ice, it's solid, water is water, it's liquid, unless the temperature exceeds the melting point of ice, ice is dry, so is this idea correct?
Faraday, who discovered the phenomenon of electromagnetic induction and laid the foundation of electromagnetism, really played with ice cubes to figure out this problem... He once experimented with two pieces of ice close together and touched them gently, and the two pieces of ice immediately froze together and were difficult to separate. Faraday concluded that the surface of the ice cube must have a liquid-like structure.
Later experiments have really found that even if the temperature is lower than the melting point, there is indeed such a layer of structure on the surface of the ice, which may be due to the vibration of water molecules and the interaction of each other. The presence of this membrane makes the ice itself slippery.
Surface structure of ice at the nanoscale, from late references[3]
However, it has all been tossed to this point, and scientists are not satisfied. A 2019 study published in the journal Nature further proved that this water film actually has a little crushed ice in it, and the viscosity, elasticity and other properties are different from water or ice, and closer to oil, which researchers believe allows ice blades to fly fast on ice [5].
Now I can finally sum up, the main reason why the ice project can be so fast, is that there is a layer of water film between the ice and the ice knife (mixed with a little ice), which greatly reduces the friction, and the source of this water film is mainly from the friction between the ice knife and the ice surface, the heat melts the ice, plus the ice surface is from the water film with the belt. It should be noted that this layer of water film is not as thick as possible, because the friction of the water film is too thick, but it will increase, so the surface temperature of the speed skating rink is usually -5 °C ~ -9 °C, not too high.
But this is not the end, from the perspective of movement, knowing the principle, there are many tricks that can be used. For example, you can slightly heat up the blade of the ice car ice sled, so that the ice can be lubricated by some water early (but the official competition will check the knife edge temperature, heating is not allowed), and for example, some lubricant can be used to increase the lubrication between the blade and the ice (international competitions on ice do not allow this, but skiing is ok to wax the skis), in addition, it is necessary to continuously optimize the geometry and materials of the ice blades in order to better create and use the water film to achieve the effect of reducing resistance.
Waxing skis | Figureworm creative
People use smooth ice surfaces to carry heavy objects, dating back to at least 7000 BC, and in order to find out why ice is so slippery, people have been studying for nearly 200 years, in this process, we can find that even if it is a common phenomenon, if you dig deep, the principle behind it is not simple at all.
Author | Ding Song
Audit | Ji Yang is a researcher at the Institute of Semiconductors, Chinese Academy of Sciences
Editor-in-charge | Ding Song
Resources:
[1] Bowden, F. P., & Hughes, T. P. (1939). The Mechanism of Sliding on Ice and Snow. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, 172(949), 280–298.
[2] Lozowski, Edward P., Szilder, Krzysztof, Maw, Sean, Morris, Alexis, Poirier, Louis, and Berni Kleiner. "Towards a First Principles Model of Curling Ice Friction and Curling Stone Dynamics." Paper presented at the The Twenty-fifth International Ocean and Polar Engineering Conference, Kona, Hawaii, USA, June 2015.
[3] Kietzig, A. M. , S. G. Hatzikiriakos , and P. Englezos . "Physics of ice friction." Journal of Applied Physics 107.8(2010):4.
[4] Blau P J . Friction Science and Technology: From Concepts to Applications, Second Edition[M]. 2008.
[5] Guoying, Bai, Dong, et al. Probing the critical nucleus size for ice formation with graphene oxide nanosheets. [J]. Nature, 2019, 576(7787):437-441.
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Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: Scientific Debunking
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