Surface Tension: Exploring Helium's Climbing Ability

[scupydog]

Hello all,

From where does the liquid helium get the energy to climb the sides of the pot that's holding it?
regards

 

[f95toli]

I guess you are referring to a Rollin film?

First of all, liquid helium at 4.2K (the stuff that is kept in a storage dewars) is just an ordinary liquid; you need to cool it down below 2.2K for it to become a superfluid (this is sometimes known as helium-II).
This distinction is quite important since superfluid is VERY different from ordinary helium; just about all properties change dramatically.

Now, superfluid He can only "creep" over the edge of a container by forming a Rollin film if it than can drop down into another He reservoir below which has a lower potential energy than the He in the pot. Hence, to answer your question: it doesn't NEED to get the energy from anywhere because the energy of the final state is lower than that of the inital state.

Now, what happens in-between (when it is creeping over the edge) is quite complicated and I will not even pretend I understand all the details. But from what I understand is this happens simply because of a very strong capillary effect; although I suspect you would need QM to fully explain why the effect is so strong in a superfluid.

 

[Savant13]

If I had to guess, I'd say it gets colder.

 

[scupydog]

I guess you are referring to a Rollin film?


Now, superfluid He can only "creep" over the edge of a container by forming a Rollin film if it than can drop down into another He reservoir below which has a lower potential energy than the He in the pot. Hence, to answer your question: it doesn't NEED to get the energy from anywhere because the energy of the final state is lower than that of the inital state.

Hi thanks for your reply, Are you saying that some how SF He knows that there is a reservoir below it!

 

[mgb_phys]

Isn't it just capillary action ?
So it comes from the free surface bonds of the material making up the container.

 

[Mentallic]

I always found superfluid helium's properties to be utterly amazing. However, since I don't have a clue why it does what it does, I have only ever thought of it as "the liquid wants to find a warmer place so it can fly"

 

[scupydog]

Isn't it just capillary action ?
So it comes from the free surface bonds of the material making up the container.

Hi, I thought capillary action was between 2 surfaces that are closer than the viscosity of the liquid in some sort of way.?

 

[mgb_phys]

No capilary action can happen where a liquid meets a surface - called a meniscus.
If you look at the surface of water in a glass it curves slightly up at the sides where it meets the glass.
Basically the energy you get by forming bonds to the surface is bigger than the energy you need to break the bonds in the liquid. Since a superfluid has zero bonds in the liquid there is no limit to how much it can form bonds with the surface.

It's the same principle as water beading on a waxed car or goretex jacket - but there you try and make a surface where the bond energies favour the water staying together in beads.

 

[f95toli]

Hi thanks for your reply, Are you saying that some how SF He knows that there is a reservoir below it!

Yes, sort of. Although the "connection" is not to the main bath (the helium reservoir beneath the pot) but to the helium film that is stuck to the outside of the pot.
The "creeping over the edge" effect only happens if the pot is initially immersed in helium and then raised (or you simply wait until the helium in the main bath has boiled off) in such a way that a Rollin film can form on the whole surface of the pot (inside AND outside).
The Rollin film will "connect" the helium IN the both with the helium that is "attached" to the OUTSIDE of the pot. When some of the helium on the outside finally forms a drop (which usually happens at the very bottom) and falls down into the main bath that leaves room for more helium, the corresponding volume is then "sucked out" of the helium left in the pot.
This goes on until there is no helium left in the pot.
If you somehow cut off the Rollin film you break this "connection" and the the helium will stop flowing.

Trivia: In old helium cryostats designed to be used with superfluid helium there was often a "film burner" for exactly this purpose; this is essentially a wire which heats up the top edge of the pot locally and destroys the film. Without the burner the helium would just creep out of the pot.
I have also seen a cryostat that quite literally had a knife edge that could be pressed down (using a long rod) and cut the film off that way.

 

[scupydog]

Thx mgb_phys And f95toli for your replies, I have some very interesting reading ahead, very interesting stuff. as another thought, what would happen in zero gravity ie an experiment on the space station, where would the drip form on the crucible, if there is a drip?or would the SF He leave the crucible as a circular blob? Not forming a rollin film .

 

[dylanesque]

I always found superfluid helium's properties to be utterly amazing. However, since I don't have a clue why it does what it does, I have only ever thought of it as "the liquid wants to find a warmer place so it can fly"

To understand this phenomenon you need to realize that it is not just the liquid which is involved but the vapour which the liquid supports above its surface. This vapour coats the walls of a container with a monolayer or so of atoms. This is completely normal and happens above all liquids. So in a container with liquid water in it, there is a monolayer of water on all the surfaces above a liquid surface.

The difference with superfluid helium is that liquid can actually flow through the ultra thin film - if it has a place to go. Its essentially the same phenomenon as siphon. With normal liquids the viscosity is so great that the rate of flow through this siphon is negligible.

All the best: Michael