Can anyone explain what a superleak is? In the context of Helium can anyone explain what a superleak is and why it could be useful?
 A: A superleak is the same as an ordinary leak (namely a hole in a container) but it has a microscopic size. Therefore no regular fluid can escape the container through this hole because its viscosity is too high. A regular fluid will rather just sit over the hole.
However, In liquid Helium-II, below the transition temperature (also called the $\lambda$ point, $T_{\lambda} \sim 2.17\,\mathrm{K}$), the superfluid phase has zero viscosity. This means it can migrate through the hole, rather easily even. A great demonstration with commentary can be found here: Alfred Leitner - Liquid Helium II the Superfluid (between 15:57 and 18:00, but the rest of the clip is definitely worth a watch as well).

As for your question about how this can be useful, I can only think of two.

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*A first use is quite simply testing for microscopic holes in a material.


*A second use is slightly more sophisticated. So-called Oscillating Superleak Transducers (OSTs) are sensors used to detect Second Sound in liquid Helium-II (or any other superfluid we may find). Second sound is basically a temperature wave in Helium-II below the $\lambda$ point. This effect is also demonstrated and commented on in the clip I linked to earlier.
If you're looking for everyday uses, the temperatures at which this effect occurs are pretty damn low, so that's not very practical.
A: As how it can be useful, to obtain ultra-cold neutrons, one can confine neutrons in a volume of superfluid Helium at around 1 K and cool them by collsions with Helium. However, you'll need to eliminate Helium 3 to avoid neutron capture. Since at this temperature, only Helium 4 is superfluid and can pass through superleak, therefore effectively eliminating Helium 3.
