IceWinter sports depend on this fact: the ice is slippery.
The low friction of the ice is the reason why speed skaters can reach 35 mph, why can figure skaters spin in circles in dizzying circles and why a 40-pound curling stone can slip and accomplish all that curling is really.
But for nearly two centuries, scientists have struggled to explain why, precisely, ice is slippery – and why skates can slip so well (or why it's so easy to slide on ice).
An obstacle: Ice skates are surprisingly difficult to study. You can not see what happens when a blade cuts through the ice, because it masks the view. And the layers of ice on which skates slide are microscopic.
Scientists must rely on their knowledge of physics and chemistry to get an explanation. They found some overlapping, each elucidating a fascinating property of ice. Since the polar vortex has settled in the Midwest this week, it's a good time to dig.
First, a reminder: What is ice cream?
Ice is solid water. You know. But what happens when it becomes solid makes this substance unusual and fascinating.
For most substances in the universe, the solid phase is denser than the liquid phase. When a material is sufficiently cooled to form solids, its molecules are bound in narrow networks. But the ice is different. When it falls below 32 ° F, special hydrogen bonds that connect water molecules force extra space between water molecules when they freeze.
On the left, the molecules of liquid water are disorganized and dense.
On the right, the ice molecules are ordered and dispersed. Wikimedia Commons
And it turns out that you can refine the ice cream for the benefit of athletes from different sports.
As Smithsonian Magazine explains, the ice used in Olympic ice rinks is purified water, sprayed on rinks one layer at a time to create surfaces of perfect consistency. The thickness and temperature of the ice at the Olympics depend on the sport. Figure skaters prefer an ice setting near 25 ° F melting point for increased grip and control. Hockey players love a cooler, harder and ultimately faster surface.
Solid ice is actually less dense than liquid water (that's why icebergs float in the ocean). And for scientists, it was a clue to understanding why the ice is so slippery.
Hypothesis 1: The pressure melts the ice. (This is usually wrong, but still interesting.)
Since the 19th century, the most common answer to the question "Why is ice cream slippery" is "because ice melts under pressure".
This idea is inspired by the work of James Thompson, who in the 1850s developed the calculation that describes a very strange property of ice: that is, under high pressure, the ice returns to the water . This is due to the fact, once again, that solid ice is less dense than water. If you squeeze the ice, it becomes less stable and melts.
You can demonstrate this effect with a very simple experience. Take a length of yarn and attach a weight to each end. Then put the thread on a large block of ice. The pressure of the wire will cut a clean line through the ice (which will freeze once the wire is passed, a process called "freezing"). See what's going on here:
It's tempting to think that this is how ice skates work: the pressure of the thin blade on the ice melts enough water to reduce friction and allow you to slide.
But here's the problem: "One would have to be an incredibly massive person to melt the ice enough to be able to skate at any reasonable temperature," said David Limmer, professor of theoretical chemistry at UC Berkeley, during the summer break. an interview last year. .
As Kenneth Chang of Good Medical explained, a person weighing 150 pounds standing on blades would only lower the melting point of the ice from 32 ° F to 31.97 ° F, while rinks for figure skating are generally kept around 24 ° F. In short, skaters can not exert enough pressure to melt the ice.
"So, although the basic idea is correct – you can melt the ice by pressurizing it – the numbers do not work at all," says Limmer. (An article explains that if the ice is very cold, it would require a pressure of 39,680 psi to melt enough to allow skating.This is more than double the pressure found at the bottom of the ocean possible to skate on the ice as cold.)
In addition, the melting pressure does not occur instantly, as you can see in the video above. "So it's inconceivable that in a millisecond a skater spends when, at a certain spot of ice, one can simply melt a layer of water," said Hans van Leeuwen, professor of theoretical physics retired, who recently published an article. mathematical explanation of ice skating, said.
Hypothesis 2: The friction melts the ice. (Warm up, but that does not explain everything.)
The pressure of a thin blade on the ice can not explain why skates slip. But what about frictions? Can not the sliding motion of the skates on the surface generate enough heat to melt the ice?
This is certainly part of the answer, but it does not explain why the ice is so unusually slippery at first. Anyone who has walked on a smooth sidewalk knows that you can slide on the ice as soon as your foot touches it. And it's not enough time to generate the friction needed to melt a film of water.
Friction "is a second order effect" in the problem of ice skating, says Limmer. Friction helps us understand why ice skates slide faster and faster when moving, but not why they can start.
Hypothesis 3: There is a very small layer of liquid water on the ice. (This is the key.)
A few years before James Thompson explained why the pressure was melting ice, the physicist Michael Faraday had discovered another fascinating property of ice: the thin liquid layer on its surface. His experience was so simple that you can do it at home.
Take two ice cubes in your freezer and very quickly – so as not to heat up some of them to their melting point – stack them on top of each other.
Come back a few hours later. They are stuck together.
Faraday (rightly) guessed that ice sticks together because of the liquid layer around them. When these liquid layers meet, they freeze together.
This very thin liquid layer also makes the ice extremely smooth. But Faraday could not prove his hypothesis at the time. The science of atoms and molecules was not yet available to help the explanation.
In 1987, scientists verified the existence of this "quasi-liquid" layer thanks to X-ray imaging. And it's super, super thin. The best estimates are that its thickness at -1 ° C is between 1 nanometer and 94 nanometers. It's about 1000 times smaller than a bacterium. More recently, scientists have actually observed the surface of the liquid using extremely sensitive microscopes.
Here is a diagram showing what happens at the molecular level.
Davide Donadio, UC Davis
When the water is frozen, the different molecules of water are seized by hydrogen bonds, staying in place in a crystalline structure, as shown in the lower part of the figure. But molecules on the surface have fewer other molecules to cling to, which makes them more disorganized – and ultimately makes the ice slippery.
Put all together
Richard Heathcote / Getty Images
So what happens when an ice skating aluminum or steel touches the ice?
Van Leeuwen explains that the tiny layer of liquid is the reason that skates can begin to move instantly on the ice. And as the blades move faster and faster in the ice, additional friction is created which melts more water. As the skater progresses, she physically travels through the ice, deforming her. This causes more friction and more melting. All of this allows skaters to slide, like a hydroplane, onto a thin, thin film of water in a channel they carve. And all this happens in an instant.
Again, all of this would be very difficult to see first-hand in an experiment. "The thickness of the water layer is so small that you can not tell it from the ice," says Van Leeuwen. So, that remains a hypothesis for the moment.
However, Van Leeuwen believes that it would be very difficult to skate at temperatures below -30 ° C. Even if there would remain a tiny layer of liquid on the ice, it would take too much friction to generate enough heat for melt anything else. In addition, below this temperature, the tiny layer of liquid above the ice becomes more and more difficult to detect. It would be like skating on gravel. (But why in the world you want to go skating at -22 ° F is beyond me.)
On what else can we skate?
D & # 39; agreement. Case closed. But I asked myself: are there other surfaces on which we could skate? As Limmer explains, "basically, all solids" will form a tiny "liquid layer" when they are close to their melting temperature ".
Mercury freezes at -37.89 ° F. It would take far too much energy to keep such a cold rink. In addition, mercury is a potent neurotoxin.
What about gallium? It's a metal melting at 85.58 ° F – a little hot for an ice rink. But imagine skaters doing triple axels on a silver mirror gallium surface. "It seems like a wonderful idea," says Limmer. (Just make sure your skids are not aluminum!) When aluminum interacts with gallium, it becomes very fragile.
Although solid gallium is more slippery near its melting point, would it be slippery enough for ice skating? Or easy enough to browse? There is only one way to find out.