# Second sound in superfluids

1. Jul 16, 2006

### armandowww

Who knows a simple way to deal with temperature wave propagation in helium II? And how about storical experimental hallmarks and Landau prediction of the second sound velocity? That's a dangerous contest for me... Thanks every reader.

2. Feb 13, 2009

### Oscur

Hey, I'm also looking for information on this topic. I did a small project reporting on the theories of second sound, but that was a couple of years ago and I didn't go into much detail at the time. I am now a physics 1st year undergrad, and I'm trying to get my head round it again. I've been referring to Tilley & Tilley (1990), but any extra help would be appreciated. I basically need to explain the concept to a group of my peers, so an explanation with a level of detail aimed at reasonably clever 18-year-olds would be good. Cheers!

3. Feb 14, 2009

### cubeleg

The liquid He4 is boson gas, so at 2.17K they condensate in a new phase called superfluid. Landau model, or two liquid model, states that the normal and the superfluid phases coexist, but below this temperature the superfluid density is bigger. This idea is the same that Landau uses to explain superconductivity, because both phases has many points in common.
The superfluid part, has two main characteristics: zero entropy and zero viscosity.

When the superfluid is heated all the superfluid is expelled of the heated region, due to its lack of entropy. This could be seen in the Fountain effect. This fact is the origin of the second sound, because when somen heat source is applied into the superfluid, it behaves like a pression wave (like sound) expelling the superfluid phase and admiting when the heat is gone. The speed of the movement depends on the density of the fluid.

I am not an expert on this topic, but I think that those are the basic concepts of the second sound. There are many other strage effects (quantization of the angular velocity, creep effect....) and the relation between superconductivity and superfluidity is very interesting, but all of this can be found in the Tilley book in great details. Anyway I hope this help.

4. Feb 15, 2009

### Oscur

Thanks for the info, you've clarified a couple of definitions on which I was a bit rusty and reassured me that Ive got the right idea. Do you or anyone else know whether there are any resources to get images of second sound happening (or animated simulations demonstrating how it works)? I'm trying to write a powerpoint on it, but google image search isn't being very friendly.

5. Feb 15, 2009

### genneth

One view which (as with all views is wrong to some degree) is that first sound is a density wave of the total fluid density, and second sound is a density wave where the total density is constant, but the relative density of superfluid to normal fluid is fluctuating.

6. Feb 15, 2009

### Oscur

This is a view I came across in Tilley and also Wikipedia. Is this something to do with a requirement that the total density of the fluid remains constant despite the thermomechanical effect? Unfortunately, I'm still not entirely clear on why a superfluid's "lacking entropy" precludes its coexisting in the vicinity of a heat source. i.e. I get *how* the normal fluid component flows away from a heat source and into a sink, while the superfluid compenent flows the other way, but not exactly why.

P.S. Sorry for thread-jacking you, O.P.!