What causes ice to sublimate?

BeechBoy

New Member
A friend of mine recently took off in a Cessna 182 with a little bit of ice on the wings :( (yeah, I told him that wasn't the best idea in the world). The flight was made at night and the temp was below freezing from the surface up through 6,000 feet (the altitude at which the flight was made).

When he landed he said that every bit of ice was gone. I understand what sublimation is but I don't know why it occurs. I thought it might be a combination of radiant energy from the sun and/or the temperature rise due to drag at higher airspeeds. Obviously, neither of these factors was present in this case. So ... what causes sublimation to occur? Is it the lower pressure at altitude? Is it the airflow?
 

tgrayson

New Member
So ... what causes sublimation to occur? Is it the lower pressure at altitude? Is it the airflow?
Evaporation occurs when a molecule has enough energy to escape the fluid or solid. Temperature is merely an average of this molecular energy. At any temperature, some molecules will have enough energy to escape, even when the average energy is low, meaning low temperature.

For instance, place a pan of water on your kitchen counter, and it will be gone in a few days. It evaporated without boiling.

Likewise, ice can sublimate at any temperature. The rate at which it will do so depends on temperature and the amount of water vapor already in the air. In dry air, the ice will sublimate readily and this is a contributing factor as to why it disappears at altitude. The relative wind ensures that the air surrounding the airplane remains dry, rather than become saturated with the evaporating water.
 

BeechBoy

New Member
In dry air, the ice will sublimate readily and this is a contributing factor as to why it disappears at altitude. The relative wind ensures that the air surrounding the airplane remains dry, rather than become saturated with the evaporating water.
Although it's only speculation because I don't know the exact environmental conditions or how much ice was on the wing but are you saying that ice which would normally take several days to sublimate will sublimate within an hour simply by exposing it to dryer air?
 

ppragman

FLIPY FLAPS!
Although it's only speculation because I don't know the exact environmental conditions or how much ice was on the wing but are you saying that ice which would normally take several days to sublimate will sublimate within an hour simply by exposing it to dryer air?
Not to mention the fact that if you apply a little friction to anything over time it will gradually get worn away. I often wonder if sublimation is a bit of a misnomer. The air provides friction. Further, the air hitting wing imparts energy into the ice which causes certain molecules of water ice on the surface to jump up to the energy needed to evaporate without melting.
 

tgrayson

New Member
are you saying that ice which would normally take several days to sublimate will sublimate within an hour simply by exposing it to dryer air?
I don't know the range within which sublimation rates will increase, whether it's 10% faster or 10 times faster. But as far as intuition goes, consider how readily large sheets of rain disappear in virga. Similar phenomenon.
 

ppragman

FLIPY FLAPS!
No, I'm saying the boundary layer means there isn't any friction at the surface, since the velocity is zero.
The very bottom of the boundary layer has zero velocity, however the boundary layer itself has velocity ranging from the freestream velocity at the top to zero at the surface of the wing. There is still friction with the surface, though, otherwise there would be no "skin friction" drag at all.

On a related note, have you ever seen mud on the upper surface of the wing? If you fly around with it on there long enough it goes away, and the sublimation of the water isn't the only part of the equation, something has to wear it away.
 

tgrayson

New Member
There is still friction with the surface, though, otherwise there would be no "skin friction" drag at all.
The friction occurs through shear forces throughout the boundary layer, due to the differential in velocity of the various layers. Since the surface is adjacent to the zero velocity, and the surface itself has zero relative velocity, the surface will feel calm air. For air to have any ability to wear something away through friction, it probably must have grid or dust embedded in the flow. Think of sandblasting.

have you ever seen mud on the upper surface of the wing? If you fly around with it on there long enough it goes away, and the sublimation of the water isn't the only part of the equation, something has to wear it away.
Skin friction isn't the only source of drag; with any sort of projection, there will be a high pressure in front of it and low pressure behind it, producing a rearward force. Mud strikes me as more likely than ice to form an uneven surface that lends itself to this sort of drag. Plus, mud is weaker in tension than ice and more likely to give way when a force is applied.

I'd be skeptical these effects would do much to a smooth coating of ice; perhaps as it begins to disperse, an uneven surface would lend itself more open to these forces, sort of like when the ice on the roof of your house drops in great sheets as it melts.
 

Realms09

Well-Known Member
Fluid phases that appear unchanging to the naked eye more specifically exist in equilibrium. Molecules are always changing state from one to the other. In the case of a puddle of water, some molecules have enough energy to leave while molecules in the air get a bit sluggish and get stuck back in the water. This happens continuously - what you might call static is actually an extremely dynamic situation. We say sublimation, evaporation, etc. occurs when the equilibrium is upset and more leave than come back. Lower pressure will make solid and liquid phases of water less stable and cause solid phase loss (easier to boil something in the mountains). Continuous exposure to dry air will also upset equilibrium. That's why you blow on soup or coffee to cool it. For a very accessible introduction to physics and atomic based thought you might want to check out Six Easy Pieces by Richard Feynman.

Regarding "friction":

Think of how a shear force can be applied to a surface. For instance, if the surface of a wing was perfectly smooth, would there be any shear force? I think not. Shear force transmission comes from molecules interacting with the "peaks and valleys" of a rough surface in a locally perpendicular or normal force manner. Bulk velocity will be zero in this region, but the individual molecules will still be bouncing around all over the place and hitting the rough surface. Some might be energetic enough to knock off the peak of the mud or ice mountain, which then gets caught up in the bulk air flow. In such a manner air can wear away a surface.

Another thought:

You could think of the air as applying a shear force on the ice, or mud, which over time causes the structure to fail (creep) and blow away. I don't know if the time scale of a flight is reasonable for this characterization. Sublimation and evaporation whould dominate, however.
 

tgrayson

New Member
but the individual molecules will still be bouncing around all over the place and hitting the rough surface. Some might be energetic enough to knock off the peak of the mud or ice mountain, which then gets caught up in the bulk air flow. In such a manner air can wear away a surface.
And this is different from evaporation....how?
 

Realms09

Well-Known Member
In one sense it's a facet of evaporation, but not a necessary component. By that I mean if there was no air evaporation/sublimation of water, mud, gold, and diamonds would still occur. Air, or any other secondary medium, acts to change the distribution of energy in the thing of interest. Air colliding with a surface can heat that surface (or mud molecule). The evaporation rate reflects that distribution.

In another sense, it is different from evaporation. Since there is a shear force present the collisions on one side of the material peak do not balance the collisions on the other. This net force imbalance manages to tear off a small group of molecules (perhaps they were hanging by a thread at the top of their mud mountain) rather than just a single molecule. In that case, there is no evaporation since the mud is still locally in the solid phase.

Regarding the original post:

Phase equilibrium diagram for H2O. Note that below the triple point the liquid phase is not stable, so water transitions directly from solid to gas.

http://www.splung.com/heat/images/phases/phasediagram.png
 

tgrayson

New Member
In that case, there is no evaporation since the mud is still locally in the solid phase.
Seems a thin distinction to me. Particles of two or three molecules probably behaves like a gas. How many do you have to have before you have a solid?
 

Realms09

Well-Known Member
Everything involving physics can eventually be termed a thin distinction since things boil down to a few basic principles. I'd say the metric is whether or not you can recognize the crystalline structure of the material.

Solids aside, when you have a gas flowing over a liquid surface you can entrain liquid droplets in the gas flow.
 
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