Why is the ice so slippery? This question is not simple
Skating, as a traditional outdoor event in winter, attracts many people to participate every year.
But have you ever wondered why the ice surface is so slippery?
Natural lubricant on ice
When asked why the ice is so slippery, some people may answer without thinking: "Because the ice surface is smooth enough!"
There may be some truth to this explanation, but anyone who has really skated on the ice must know that the outdoor ice surface is often rough and not smooth at all.
But the strange thing is that if you are not paying attention, you can fall on the ice.
Needless to say, the outdoor ice rink, even if it is a pile of broken ice on the side of the road in winter, it can instantly knock people down.
Another example can also prove that such an explanation is unreasonable.
The clean marble floor in the hotel is much smoother than the natural ice surface, but people can still walk on it like flying without being as careful as on the ice surface.
So, what makes the ice so smooth?
In fact, if you want to make an object slippery, you only need a thin layer of lubricant. It can be water, wax, mineral oil, or fine sand or graphite powder.
Lubricants can greatly reduce the friction between objects.
Water is the natural lubricant that comes with ice.
When we stand on an ice surface, our feet are not in direct contact with the ice surface, but are separated by a fluid water film.
Where does the water on the ice come from
Speaking of this, the question of why the ice surface is slippery has been explained, but there is still an unsolved problem-where does the water on the ice surface come from?
We all know that ice will only melt at zero degrees Celsius.
The outdoor temperature in Beijing reached minus five or six degrees last month, and it was even as low as minus ten degrees. This temperature is obviously not enough to melt the ice surface.
In fact, "ice melts at zero degrees Celsius" is not accurate. It ignores an important premise, that is, only under a standard atmospheric pressure, the melting point of ice is zero degrees Celsius.
As early as 1850, scientists discovered that increasing pressure would lower the melting point of ice.
The first person to associate this rule with skating was Irish physicist John Jolly.
He believes that when a person stands on an ice surface, the pressure generated by the body on the ice surface will make the melting point of the ice below zero degrees Celsius, which will melt the ice surface and form a lubricating water film, which will make the ice surface slippery. .
If a person wears skates with thin ice skates, the pressure on the ice will be greater and the melting point will drop more.
According to John Jolly's calculations, an adult wearing skates will apply a pressure more than 400 times the standard atmospheric pressure on the ice surface, which can reduce the melting point of the ice to -3.5 degrees Celsius.
The question of "where does the water on the ice come from" seems to be perfectly solved, but there is always something wrong.
After the winter, the temperature in Northeast my country can reach tens of degrees below zero. According to John Jolly's theory, the ice in the Northeast should not melt, make people slip, and it is difficult to slide on it.
However, this is not the case. "Sliding" on the ice is one of the daily winter activities in Northeast China.
Not to mention the northeast, the best temperature on the ice surface of the general figure skating rink is about -5.5 degrees Celsius, and the skates on the figure skating shoes are very thick. In most cases, athletes are as light as a yan. John Jolly’s theory is completely It cannot be explained.
Even more embarrassing is that in his experiment, John Jolly set the contact area between the skate and the ice surface to be 0.02 square inches.
But in fact, ice skates are very thick, and the contact area is much larger than this number, which means that the pressure exerted on the ice surface may not be enough to make the melting point drop so much.
Someone once calculated that a 70 kg person standing on ice skates can only reduce the melting point of ice by 0.017 degrees Celsius.
In 1939, Frank Bowden and T.P. Hughes proposed that the heat generated by the friction between the sole or ice skates and the ice surface caused the ice surface to melt and form a water film.
After repeated experiments, they believe that friction is the main factor that melts the ice surface compared to pressure.
In order to verify this conclusion, they even set up the experiment site in an icy cave at an altitude of 3346 meters, and used refrigeration technologies such as dry ice and liquefied gas.
However, their explanation did not satisfy the academic community.
At this time, a thought arises in people's minds: Is there a layer of water on the surface of the ice itself?
In the middle of the 20th century, with the advancement of solid state physics and material science, as well as the renewal of material detection technology, people discovered that a layer of surface material with fluidity between water and ice naturally exists on the surface of ice.
It is not water formed after ice melts. Even if the temperature is lower than the melting point, this layer of material can exist. Some scholars call it the "quasi-liquid layer."
Its thickness is related to temperature. The more the temperature is below the melting point, the thinner the quasi-liquid layer.
In 1987, scientists used X-ray diffraction to "see" the microstructure of the ice surface and confirmed that there was indeed a layer of quasi-liquid on the ice surface.
In the 21st century, scientists have used nonlinear optics to further study the structure and properties of quasi-liquid on ice.
Some research teams used molecular dynamics methods to use computers to simulate the arrangement of water molecules on the ice surface, and even simulated how water molecules "run around" on the ice surface.
But are these water molecules a lubricant?
Lubricants in life are usually oily and paste-like, and texture like water is rarely seen.
Lubricants must not only have good fluidity, but also usually have a certain degree of elasticity and viscosity.
A study published in the journal Nature showed that scientists used a tuning fork-like device and millimeter glass beads to cleverly measure the mechanical properties of this layer of quasi-liquid.
The vibration of the tuning fork drives the small glass beads to slide on the ice surface, and the accelerometer captures the movement of the glass beads in the process.
Scientists have discovered in experiments that this layer of quasi-liquid can be seen as a mixture of water and tiny crushed ice. It is much more viscous than water, like oil, and has the same elasticity as solid ice.
Appropriate viscosity and elasticity make the quasi-liquid on the ice surface even better lubricating effect than water, which explains why the ice surface is slippery than a smooth floor splashed with water.
In this regard, the question of "why is the ice slippery" has a more complete explanation.
Having said so much, many people must have been unable to restrain themselves and want to go to the ice rink to show off their style immediately.
Don't worry, finally remind everyone that you must skate on the regular ice rink, wear the necessary protective gear and pay attention to your own safety.
(Source: Digital Beijing Science Center)