Naked Science Forum
General Science => General Science => Topic started by: thedoc on 25/10/2011 17:15:50
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Why are ice and snow slippery?
Asked by Simon
Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2164/)
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We answered this question on the show...
Chris - That's a very hard to answer question actually! In fact, it’s the source of a lot of disinformation.
If you have a look in many, many texts and books and things, especially historically, you will find they claim that when you have an ice skate, for example, on an ice rink, the pressure of the ice skate pushing down on the ice causes the ice to increase its temperature because it’s being compressed and it melts a bit, and this puts a layer of water on the surface of the ice and that means the ice skate then slides along.
This doesn’t stand up to scrutiny though because people wearing shoes - where the same weight is distributed over a much bigger area therefore with much lower pressure - still slip over on the ice!
In fact, if you plug the numbers into the calculations, you'll find it actually only changes the temperature of the ice by a fraction of a degree. So this cannot account for why ice is slippery. It can't be a melting phenomenon.
What scientists actually think is going on is that probably because you have an ice surface and there’ll be water molecules on the surface which are not tethered to other water molecules, they exist more as a liquid than as a solid. So, all ice is covered in a very thin, at a molecular level, layer of water. Michael Faraday showed this quite convincingly because he got two ice cubes, which are both slippery, put them together and they stuck. Now, if there wasn’t water there, then they wouldn’t have stuck together.
So what scientists think is going on is that there is a very thin layer of water on the surface of any ice surface and it’s that water that acts as the lubricant and reduces the level of friction at the surface. So, I think that's probably the best explanation I can come up with. And even at very low temperatures, when something moves across the surface, friction between the moving object and the ice causes heating and rapidly produces the liquid layer.
Dominic - Does that depend on the temperature of the ice?
Chris - Well, to a certain extent it will, but at the same time if you've got molecules of water which are sitting on the surface of the ice which are untethered and unbonded to other surrounding molecules then it’s easier for them to have energy and detach and form a water film than it is for molecules elsewhere in the ice to move around. So, it favours the formation of a layer of water at that point, which could make them slippery.
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try rotating 1 ice cube & see if they stick? i dont got ice cubes
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For balance (not the standing on skates sort) I thought a link to the ongoing thread in which Damocles and Geezer are mounting a fine rear-guard action for the old orthodoxy that it is pressure rather than anything else that causes the layer of liquid
http://www.thenakedscientists.com/forum/index.php?topic=41684.msg370647#msg370647
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For balance (not the standing on skates sort) I thought a link to the ongoing thread in which Damocles and Geezer are mounting a fine rear-guard action for the old orthodoxy that it is pressure rather than anything else that causes the layer of liquid
http://www.thenakedscientists.com/forum/index.php?topic=41684.msg370647#msg370647
if ice forms from the outside in, how can the outer layer be liquid?
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OK then Imaatfal I am well and truly incensed now! The disinformation in this case is all on the part of the show!
Chris - That's a very hard to answer question actually! In fact, it’s the source of a lot of disinformation.
If you have a look in many, many texts and books and things, especially historically, you will find they claim that when you have an ice skate, for example, on an ice rink, the pressure of the ice skate pushing down on the ice causes the ice to increase its temperature because it’s being compressed and it melts a bit, and this puts a layer of water on the surface of the ice and that means the ice skate then slides along.
This doesn’t stand up to scrutiny though because people wearing shoes - where the same weight is distributed over a much bigger area therefore with much lower pressure - still slipover on the ice!
In fact, if you plug the numbers into the calculations, you'll find it actually only changes the temperature of the ice by a fraction of a degree. So this cannot account for why ice is slippery. It can't be a melting phenomenon.
What scientists actually think is going on is that probably because you have an ice surface and there’ll be water molecules on the surface which are not tethered to other water molecules, they exist more as a liquid than as a solid. So, all ice is covered in a very thin, at a molecular level, layer of water. Michael Faraday showed this quite interestingly because he got two ice cubes which are both slippery, put them together and they stuck. Now, if there wasn’t water there, then wouldn’t have stuck together because obviously, then the water then froze onto both.
So what scientists think is going on is that there is a very thin layer of water on the surface of any ice surface and it’s that water that access the lubricant and reduces the level of friction at the surface. So, I think that's probably the best explanation I can come up with.
(my emphasis)!
The whole point is that the weight is not distributed over a larger contact area if you are wearing shoes than skates, because both shoe and ice have rough surfaces on the microscopic scale, and the actual area of contact is much smaller than -- a minute fraction of -- the apparent area of contact for shoes or for skates, and exactly the same in both cases. That is why Amonton's Law for static friction involves only the weight, and not the area of contact -- it is the weight that determines the actual area of contact for any material!
"So what scientists think is going on ..." -- the scientists who think that is what is going on are not those who work and specialize in the friction/lubrication area of surface science!
When you stand on ice, the initial actual contact area is much smaller than a square millimetre. But the ice at those few points of contact undergoes pressure melting, and your shoe or skate gradually comes into a larger contact area of thin liquid film over ice as the melting proceeds to accommodate your weight. You are then in a good position to aquaplane.
It is only when you start to move that frictional heating, which is a significant factor in the whole process, can come into play. No motion, no frictional energy dissipation, and no heating.
The reference I have given in the other thread provides a good authoritative treatment of the subject, but if anyone really believes that this is an example of "many, many texts and books and things, especially historically ..." which have perpetrated an error, I can only point to more recent research articles in this specialist research literature, which are fairly easily located with a google search. Adamson does not merely assert, he discusses measurements and evidence that lead to the conventional surface chemistry picture which is:
For friction on most material surfaces: yield failure and plastic flow to increase the area of initial contact until the interface pressure matches the yield pressure of the softer material: Actual area of contact = normal (meaning perpendicular) force / yield strength of softer material. Maximum force of static friction = shear strength x actual area of contact.
For friction on ice: pressure melting to produce a liquid film until surface ice temperature matches melting pressure. Little force of static friction, with the only real frictional barrier being viscous drag in a very thin lubricating film of liquid water. Pressure melting can only occur for the very few materials whose solid phase is less dense than the liquid phase.
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For balance (not the standing on skates sort) I thought a link to the ongoing thread in which Damocles and Geezer are mounting a fine rear-guard action for the old orthodoxy that it is pressure rather than anything else that causes the layer of liquid
http://www.thenakedscientists.com/forum/index.php?topic=41684.msg370647#msg370647
if ice forms from the outside in, how can the outer layer be liquid?
Because ice also melts from the outside in.
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OK then Imaatfal I am well and truly incensed now! The disinformation in this case is all on the part of the show!
Matt, I think you really need to bring this to Chris's attention (cos I'm keeping out of it.)
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Ice and snow are slippery because of its melting point.When you step on ice, you increase the pressure on the ice so because of the pressure of your feet or body melting and creating that thin film and it makes you slippery on Ice.