In the Beijing Winter Olympics Figure Skating Team Ice Dance Free Dance Competition on February 7, Chinese athletes Wang Shiyue and Liu Xinyu danced on the ice | Source beijing2022.cn/
Editor's Note
On February 4, Beijing time, the Beijing Winter Olympics officially kicked off. On the day of the opening ceremony, there was a wonderful figure skating team competition, and in the subsequent figure skating women's singles and men's singles competitions, we also enjoyed the athletes' stunning posture on the ice.
When watching ice sports such as curling, ice hockey, short track speed skating, figure skating, etc., have you ever wondered why the ice surface is so "silky"? Is frictional heat causing the ice to melt, or is it that the pressure increases when a person stands on the ice and melts? These two are probably the answers that most readers think.
Wu Jinyuan, a doctor of physics, pointed out that the question about why the ice surface is slippery is actually not simple, and the answer may be more interesting than you think.
Written by | Wu Jinyuan
Editor-in-charge | Chen Xiaoxue Wang Yudan
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Water freezes at 0 degrees Celsius, which is a common sense that everyone has. However, many readers will immediately point out: not necessarily. Pure water, at an atmospheric pressure, has a freezing point (i.e., freezing point) of 0 degrees Celsius. If the water is impure and dissolves other substances, its freezing point will be reduced. For example, sprinkling salt on the road after snow in winter can prevent the road from freezing. Another factor that changes the freezing point is the pressure, the greater the pressure, the lower the freezing point.
01 "Pressure changes melting point"
When external pressure is imposed on an object, the molecules inside it tend to be pressed more "porcelain", or denser. For most substances, the solid state is denser than the liquid state, so pressurization will make most substances tend to change from liquid to solid. We can roughly imagine pressurizing a finely crushed cinder into a briquette. The case of water is more special, in a certain pressure and temperature range, the density of the liquid state is greater than the solid state, so the pressure tends to make the ice into water. It's a bit like pressurizing a brick house (solid ice) where the connection between the bricks is broken and eventually becomes a smaller pile of bricks (liquid water).
However, people often have some misunderstandings about the change of the melting point of pressure. In some publications, this statement can be seen: "When two pieces of ice below 0 degrees Celsius are pressed together, you can see that the ice at the contact melts into water, removes the pressure, and the water solidifies into ice, resulting in two pieces of ice forming one ice." In addition, some authors believe that the accumulation of loose snow particles in their hands is also due to the increase in pressure causing the ice to melt into water at a lower temperature and then freeze into clumps.
The increase in pressure lowers the melting point of the ice, which is right. But the explanation for these two phenomena is problematic: usually, squeezing ice or snow masses by hand creates a relatively small pressure on the surface of ice and snow particles, far from enough to lower the melting point of the ice enough to melt.
02 Experiment and puzzle solving
I did an experiment in which two pieces of ice were combined into one piece of ice.
One ice cube is placed on the table, and then we stack another one on top of the ice cube. After the ice cubes are folded, gently press them to squeeze out as much water as possible between the ice cubes. After a few seconds like this, when we pick up the ice cubes on the upper layers, we can see that the ice cubes on the lower layers have frozen together with the ice cubes on the upper layers.
In this experiment, we didn't press two pieces of ice hard, so the reason the two pieces of ice merged into one piece of ice in this experiment was not because the pressure caused the ice to melt and then solidify.
We did this experiment at room temperature, in which case the ice cubes are already surrounded by a layer of water, as shown in the following figure:
Figure 1 Contact and freezing of ice cubes. In the figure, light blue represents liquid water and dark blue represents solid ice
When the two ice cubes are put together, most of the water between them is squeezed away, because the center of the two ice cubes is below 0 degrees Celsius, the water between them is sandwiched between the ice cubes, and there is no heat supplement from the outside air, and soon the temperature drops below 0 degrees Celsius, so it freezes into ice.
So can the experiment mentioned earlier in the book that describes the melting of ice caused by pressure be done? Maybe yes, but not at room temperature, because the phenomena seen at room temperature have nothing to do with pressure melting.
For example, if we do this experiment in the winter -1 degree Celsius environment, how much pressure is it needed to make the ice melt at -1 degree Celsius? About 100 atm. When the contact surface of ice is 1 square centimeter, it requires about 1000 newtons of force, equivalent to 100 kilograms of weight (one kilogram of object subject to the gravity of the Earth is 9.8 newtons, this article approximates this number to 10 newtons). So this experiment is actually very difficult to do. Who can produce such a large force with their fingers? Do you want to invite a zen martial monk from the shaolin temple who is very skilled to try it?
03 Is it reliable to explain skating with the pressure melt theory?
Many readers will say that when we were in school, when skating, the ice knife and the ice surface contact placed a high pressure, so that the ice melting point of the contact point decreased, turned into water, played a lubricating role, so that the friction became very small.
This theory is called the pressure melt theory, and it has been around for more than a hundred years. In recent years, however, there has been a great deal of skepticism about this theory.
First of all, in order to melt the ice under the ice blade, a very high pressure is required. Usually, during figure skating competitions, the temperature of the ice surface is at least below -3.5 degrees Celsius. When short track speed skating, ice hockey and other games, the ice surface temperature is lower. For the ice to melt at -3.5 degrees Celsius, the pressure required is about 350 atmospheres, or 35 million newtons per square meter, or the equivalent of stacking 3500 tons of heavy objects on an area of 1 square meter.
Of course, it is not as difficult to produce this 350 atmosphere pressure as we intuitively think, if we assume that the effective length of the ice knife is 100 mm and the width is 0.5 mm, then its effective area is 50 square millimeters. To achieve 350 atmospheres, athletes need to weigh up to 175 kilograms.
You may ask, are there skaters with a weight of 175 kilograms? I have the same doubt, you know, we're only calculating the weight needed to melt the ice at -3.5 degrees Celsius, and if the ice is cooler, it seems that we have to go to weightlifters or sumo wrestlers to pick out the figure skating seedlings.
In fact, however, we didn't see any difficulty with figure skaters who were as light as a swallow. Consider a more extreme example: if we put a pebble on the ice and kick it away with our feet, the stone will also slide far. The contact area between the stone and the ice surface is relatively large and the weight is not large, so the pressure on the ice surface is far less than a few hundred atmospheres. It can be seen that pressure melting should not be the main mechanism that makes the ice slippery, and there must be other mechanisms.
One of the strongest counter-proofs of the pressure melt theory is that ice is still slippery when it is below -20 degrees Celsius, and that in such a cold environment, people can still skate. When the water is below -20 degrees Celsius, no matter how it is pressurized, it is still ice.
04Why is the ice slippery when skating?
In recent years, two other mechanisms have gradually been discovered, namely the surface melting mechanism and the friction melting mechanism.
Friction melting mechanism, refers to the ice knife on the ice surface quickly slipped over, friction generated heat so that the ice under the ice blade melted. Perhaps this mechanism will play a bigger role in short-track speed skating.
The surface melting mechanism means that there is always a thin layer of liquid-like water on the surface of the ice, and even below the freezing point, it does not completely freeze into ice. Like a rotten brick house, there is always a mess of bricks (i.e., water that has not crystallized into solid water) outside the walls. From Figure 2, we can see that inside the ice cube, water molecules are regularly distributed according to a certain spatial structure, and crystallize into a solid state. On the surface of the ice cube, water molecules randomly gather together, and each molecule is easily moved after being subjected to external forces, thus forming a thin layer similar to a liquid.
Figure 2 Schematic diagram of melting the surface of the ice cube
Since this layer of water is always present, when the object comes into contact with the ice surface, it does not need too high a pressure, so that the water can continuously lubricate the contact surface of the object with the ice, so that the ice becomes slippery.
At present, the physics community is more inclined to agree that the surface melting mechanism plays a greater role than the pressure melting mechanism. On the one hand, microscopically speaking, there is indeed various experimental evidence to confirm that below the freezing point, there is an uncrystallized, liquid-like water molecular layer on the surface of the ice, and its thickness is about 10 to 100 nanometers. On the other hand, in the macro world, we see people skating in environments well below minus 20 degrees Celsius, and we also know that losing weight doesn't cause skaters to perform less. These phenomena are more supportive of the surface melting mechanism.
Interested readers can read the following references:
http://lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf
http://dujs.dartmouth.edu/2013/04/what-causes-ice-to-be-slippery/#.WTr8HE-GNR4
Plate editor| Ginger Duck