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Author Topic: Why are ice and snow slippery?  (Read 4761 times)

simon

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Why are ice and snow slippery?
« on: 24/10/2011 01:30:03 »
simon  asked the Naked Scientists:
   
I have a question for the Naked Scientists......

We all know that ice and compacted snow is slippery, but why is this?

Fantastic show by the way.

Simon

What do you think?
« Last Edit: 24/10/2011 01:30:03 by _system »


 

Offline Geezer

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Why are ice and snow slippery?
« Reply #1 on: 24/10/2011 09:45:55 »
Ooooo! I remember this one. I got beaten up so badly the last time that I'm keeping schtum.
 

Offline damocles

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Why are ice and snow slippery?
« Reply #2 on: 24/10/2011 14:15:06 »
Ooooo! I remember this one. I got beaten up so badly the last time that I'm keeping schtum.

 [V] Oh Geezer! You might at least have included a link to "the last time" to let us newcomers know where the bear traps were laid!

 :-\ I am enough of a fool mad scientist to rush in anyway and provide an authoritative-sounding lecture on the subject.

Firstly, ice (and compacted snow) is only really slippery close to its melting point. It becomes much less so below about -20°C, and below -40°C skiing and skating become almost impossible, and special waxes are needed. Water is unusual among materials in that its solid form, ice, is less dense than liquid water. This means, among other things, that the melting point is lower at higher pressures (follows from Le Chatelier's principle), and that when someone stands on ice close to its melting point, the local pressure will cause enough melting to produce a thin film of liquid water. This film can cause efficient hydrodynamic lubrication (aquaplaning), and once sliding starts, both pressure and frictional heating between them will ensure that a newly forming film of liquid water will follow the motion to its end. Because of lack of pressure and contact with ice below its freezing point, the film will rapidly re-freeze behind the motion. Skating and skiing both rely on this aquaplaning mechanism, but skates have blades and skis have hard edges to cut into the ice (which is really a very soft solid) and control the direction and extent of the motion.
 

Offline mitch

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Why are ice and snow slippery?
« Reply #3 on: 24/10/2011 14:18:16 »
Regelation. Ice isn't slippery until it's wet but it does melt under high pressure and heat. So the layer of water is what causes it to be slippery. I would imagine the same is true for snow.
 

Offline chris

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Why are ice and snow slippery?
« Reply #4 on: 24/10/2011 17:16:03 »
The answer my research turned up is that the slipperiness of the surface occurs owing to a water / ice interface; this is generated by friction between the contacting surface and the ice; the friction generates sufficient heat to liberate some of the less-tightly bound water molecules at the ice surface, producing the water layer; this, in turn, lubticates the passage of the contacting surface over the ice surface.

In most practical circumstances, the answer is NOT owing to pressure effects altering the melting temperature. The calculations show that, even beneath an ice-skate blade supporting the weight of an adult, the alteration in melting temperature is only a fraction of a degree and therefore cannot account for the low friction.

Chris
 

Offline Geezer

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Why are ice and snow slippery?
« Reply #5 on: 24/10/2011 23:06:20 »

The calculations show that, even beneath an ice-skate blade supporting the weight of an adult, the alteration in melting temperature is only a fraction of a degree and therefore cannot account for the low friction.


The calculations may show it, but it still doesn't add up. If that was true, if you stood still while standing on skates, you'd have a devil of a time getting going again because your skates would be effectively frozen on to the ice. Not only that, but the well known "falling flat on your arse" effect when the skates zoom out from underneath you when you are trying to stand still would be highly improbable.

I have to wonder if the people who came up with this theory even bothered to get up on a pair of skates.

It's the same on skis. The only serious injury I ever got while skiing was when I was (trying to) stand still while waiting for someone on a flat bit near the bottom of a chair lift.

(Oh crap! I got sucked into this one again.) 
 

Offline damocles

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Why are ice and snow slippery?
« Reply #6 on: 24/10/2011 23:47:35 »
The answer my research turned up is that the slipperiness of the surface occurs owing to a water / ice interface; this is generated by friction between the contacting surface and the ice; the friction generates sufficient heat to liberate some of the less-tightly bound water molecules at the ice surface, producing the water layer; this, in turn, lubticates the passage of the contacting surface over the ice surface.

In most practical circumstances, the answer is NOT owing to pressure effects altering the melting temperature. The calculations show that, even beneath an ice-skate blade supporting the weight of an adult, the alteration in melting temperature is only a fraction of a degree and therefore cannot account for the low friction.

Chris

So that is the trap! Sorry to have to say so, Chris, but your research is quite wrong in its conclusion and Geezer's observation is quite correct.

The answer IS connected to contact melting. Frictional heating can only account for low dynamic friction, not for low static friction. Relevant observations that back up my view are
  • (1) Geezer's observation of low static friction
  • (2) The fact that other low-melting solids usually do not show slipperiness to nearly the same degree that ice does
  • (3) The grippiness (best word I could think of) of low temperature ice, and watching videos of people trying to ski on really cold icy snow doing a lot of preliminary kicking to get themselves started
  • (4) Text that you can read in a lot of Surface Science/Physics/Chemistry research journals and respected textbooks on surface science.**

The issue is that the "calculations" you cite are based on the notion that pressure = weight / contact area, and that the area of contact between a person and the ice/snow is around 1 sq dm for skis or shoes, and several sq cm for skates.

But in fact, the surface of ice/snow and of boots/skis/skates is rough and irregular on a microscopic/sub-microscopic scale, and the actual area of initial contact is less than 1 sq mm. So the pressure is quite adequate to force an initial melting at the minimum freezing point of water (= -22°C) and to continue to melt water until enough of a film is formed to create the conditions I described in my previous post.

------
** PS Not really fair of me to put something like this in without a citation. Have now checked my library. I suggest that Chapter XII of Adamson & Gast: "Physical Chemistry of Surfaces" would be a good place to start.
« Last Edit: 25/10/2011 22:09:53 by damocles »
 

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Why are ice and snow slippery?
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