Just recently I discovered the world of the superfluids and I am

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Hut
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Just recently I discovered the world of the superfluids and I am astonished by the characteristics of Helium II. Even after researching for hours there a still a few questions which I wasn’t able to answer myself.

Rollin Film:
If a container is filled with helium II are the walls and the ceiling of this container going to be covered without any further impact from the outside after a few hours have passed by? How does this phenomenon go along with the law of the conservation of energy? The liquid climbs in the container so it gains potential energy.

For the fountain effect to happen you have to heat up the superfluid in order for it to change back to the helium I state.
How can you heat up a superfluid if is capable of transporting heat like waves to other places (“2nd sound”) ?

Thanks in advance.

Regards
 

Answers and Replies

  • #2
Bill_K
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The liquid climbs in the container so it gains potential energy.
Capillary attraction between the film and the container wall reduces the energy.

How can you heat up a superfluid if is capable of transporting heat like waves to other places (“2nd sound”) ?
Second sound, also called temperature waves or entropy waves, is a density fluctuation in the gas of phonons. Its propagation requires a sufficient amount of energy-conserving phonon-phonon interactions. You can generate a wave of second sound with a temperature pulse. Heating by comparison is like a d.c. signal.
 
  • #3
Hut
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Thanks for your quick answer.

Capillary attraction between the film and the container wall reduces the energy.
But that would also mean that the capillary attraction gets stronger the higher the container is. I am having a hard time picturing that.

Since I couldn't find enough reading material regarding the rollin film I don't know how to to imagine at.
If the Helium II sits at the bottom of a container will it creep up walls until everything is fully covered by a 30nm thick layer?
 
  • #4
f95toli
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If the Helium II sits at the bottom of a container will it creep up walls until everything is fully covered by a 30nm thick layer?
No, he helium can only "escape" if there is already a film covering the wall AND the film is continous on the outside wall all the way down to a point BELOW the starting point.

If you do this experimentally (I used to to this experiment when I was a TA) you normally use a small "bucket" which is first dunken in the superfluid, this ensures that there is a film covering the whole bucket, when you lift the bucket out of the liquid any helium that is still inside the bucket will start to creep out and form droplets at the bottom of the bucket, which then falls back into the main bath.
 
  • #5
Hut
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Thanks for your answer f95toli.

Te captation of Fig.4 on the wikipdia page states following (http://en.wikipedia.org/wiki/Superfluid_helium-4)

Fig. 4. Helium II will "creep" along surfaces in order to find its own level – after a short while, the levels in the two containers will equalize. The Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.
Since you said that:
helium can only "escape" if there is already a film covering the wall AND the film is continous on the outside wall all the way down to a point BELOW the starting point.
.
Does ist mean that the extract from the wikipadia page is simply wrong or is it just misleading? I am wondering because then there would be no reason for "the Rollin film to cover the interior of the larger container".
 
  • #6
f95toli
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Does ist mean that the extract from the wikipadia page is simply wrong or is it just misleading? I am wondering because then there would be no reason for "the Rollin film to cover the interior of the larger container".
No, what I wrote agrees with the wiki. In the illistration the level inside the bucket is lower than the outside so helium will flow INTO the bucket, I described a different situation with a bucket ABOVE the main bath which means that the liquid will LEAVE the bucket.

Also, the situation described in the wiki does require there to be a He film to start with, which tends to be the case in just about every practical situation you can think of. If you separate the film into two nothing can flow.
The Rollin film can be a problem when designing cryostats (although it is rarely a problem in modern designs), and then heaters or knifeedges are used to "cut" the film into two at the top of the bucket in order to prevent the He from escaping.
 

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