Gas Viscosity

[MedPR]

---

Members do not see this ad.

Why are gases more viscous at higher temperatures?

 

[411309]

"james maxwell published a famous paper in 1866 using the kinetic theory of gases to study gaseous viscosity.http://en.wikipedia.org/wiki/Viscosity#cite_note-Maxwell1866-12 To understand why the viscosity is independent of pressure, consider two adjacent boundary layers (A and B) moving with respect to each other. The internal friction (the viscosity) of the gas is determined by the probability a particle of layer A enters layer B with a corresponding transfer of momentum. Maxwell's calculations showed him that the viscosity coefficient is proportional to both the density, the mean free path and the mean velocity of the atoms. On the other hand, the mean free path is inversely proportional to the density. So an increase of pressure doesn't result in any change of the viscosity."

so basically at higher temperatures theres a greater velocity/mean free path which means greater viscosity which this dude basically determined experimentally.

 

[MedPR]

"james maxwell published a famous paper in 1866 using the kinetic theory of gases to study gaseous viscosity.http://en.wikipedia.org/wiki/Viscosity#cite_note-Maxwell1866-12 To understand why the viscosity is independent of pressure, consider two adjacent boundary layers (A and B) moving with respect to each other. The internal friction (the viscosity) of the gas is determined by the probability a particle of layer A enters layer B with a corresponding transfer of momentum. Maxwell's calculations showed him that the viscosity coefficient is proportional to both the density, the mean free path and the mean velocity of the atoms. On the other hand, the mean free path is inversely proportional to the density. So an increase of pressure doesn't result in any change of the viscosity."

so basically at higher temperatures theres a greater velocity/mean free path which means greater viscosity which this dude basically determined experimentally.

I get that higher temp = more KE = more collisions, but doesn't it also mean the collisions occur at a higher energy so the colliding particles are less likely to stick to each other? I don't understand how a low temperature gas, which basically makes it right on the verge of becoming a liquid, can have a lower viscosity than a high temp gas, which is the furthest thing from a liquid.

 

[411309]

That was my initial thoughts. I thought it would be inversely proportional to temperature because cold air would be more dense and harder to transverse. But I guess im wrong. I guess more collions at a rapid rate makes it more viscous. In fairness, I live in florida and sometimes when its humid the air does feel kinda "thick", for lack of a better word, lol.

 

[MedPR]

That was my initial thoughts. I thought it would be inversely proportional to temperature because cold air would be more dense and harder to transverse. But I guess im wrong. I guess more collions at a rapid rate makes it more viscous. In fairness, I live in florida and sometimes when its humid the air does feel kinda "thick", for lack of a better word, lol.

Yea, but humidity is about moisture in the air and isn't related much to temperature (if at all). I'm from a pretty humid region too, but I've also been to las vegas quite a few times and even though it gets way hotter there than where I live, it is never as humid. 😕

 

[pfaction]

Also I would have said decreased viscocity: Temp proportional to KE, and gases have low (if any) intermol interactions - increasing temp gives them the ability to fight off those interactions by definition.

 

[MedPR]

Also I would have said decreased viscocity: Temp proportional to KE, and gases have low (if any) intermol interactions - increasing temp gives them the ability to fight off those interactions by definition.

Yea, but increasing the KE makes them collide more. So it increases the amount of interactions, though I still think it reduces the length of those interactions.

Anyone have a definitive reasoning why temp up = viscosity up for gases?

 

[fluoropHore]

can i see where youre reading this from? is this before supercritical temp/P?

 

[milski]

Viscosity is technically the ability of a fluid to transfer momentum between its particles. It is not really about particles "sticking" together - in gas that just does not happen, they collide and go away from each other.

A dense gas carries more more momentum but since the particles are closer to each other they're able to carry it over less distance. The molecules do move faster when T is higher and that does result in higher momentum transfer, aka higher viscosity.

The stickiness between molecules does play role when talking about liquids and is the reason why viscosity of fluids increases when the temperature is lowered.

 

[MedPR]

Viscosity is technically the ability of a fluid to transfer momentum between its particles. It is not really about particles "sticking" together - in gas that just does not happen, they collide and go away from each other.

A dense gas carries more more momentum but since the particles are closer to each other they're able to carry it over less distance. The molecules do move faster when T is higher and that does result in higher momentum transfer, aka higher viscosity.

The stickiness between molecules does play role when talking about liquids and is the reason why viscosity of fluids increases when the temperature is lowered.

Ah that makes sense, I didn't think (or know) about the momentum aspect. Thanks again.