Aerogels are materials that are like ~90% or more air. As I understand, the topology of the material (i.e. of that part of the aerogel that is not air) is not such that air is contained into bubbles. Rather, the structure is similar to that of a "jungle-gym" (see this this answer).

Now, it is possible to soak aerogels in a superfluid like Helium-II or He-3. As I understand it is possible to make the aerogel absorb the superfluid in two different ways:

  • "Partially filled": the superfluid does not fill all the vacuum contained into the aerogel, but it sticks to the internal "aerogel surface", namely it covers the topologically intricate surface of the "jungle-gym" structure (i.e. we have a superfluid film on a complex substrate, a system first studied by Reppy and collaborators1). The superfluid domain is 2D.

  • "Completely filled": the superfluid fills all the vacuum space that constitutes the "pores". The superfluid domain is 3D but it is not simply connected (see e.g. this article2).

My question is: why are these systems studied and interesting and what are we supposed to learn from these kinds of experiments? Is there any difference between porous media "partially" (or "completely") filled with Helium-4 and Helium-3?

  1. J. E. Berthold, D. J. Bishop, and J. D. Reppy, "Superfluid Transition of $\require{mhchem} \ce{^4He}$ Films Adsorbed on Porous Vycor Glass", Phys. Rev. Lett. 39, 348 (1977).
  2. G. K. S. Wong, P. A. Crowell, H. A. Cho, and J. D. Reppy, "Superfluid critical behavior in $^{4}\mathrm{filled}$ porous media", Phys. Rev. Lett. 65, 2410 (1990).

A news article from Cornell in 1996 suggests aerogels are used to modify the properties of superfluid Helium without destroying superfluidity. For comparison, submerging superfluid helium in powders and other porous media tends to destroy superfluidity through various mechanisms


  • $\begingroup$ Many thanks, the article is really interesting. The original research articles are too difficult for me at the moment, but this gives some nice ideas. I have the feeling that most of the literature is from the 90's, I wander if the topic lost some of its attractiveness (and why) or there are also recent reviews. $\endgroup$ – Quillo Aug 25 '20 at 21:28
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    $\begingroup$ @Quillo There was a "heated" controversy about supersolids: newscientist.com/article/… $\endgroup$ – user137289 Aug 25 '20 at 21:50
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    $\begingroup$ @Quillo I know there was at least one group at caltech using liquid helium as a highly sensitive gyroscope that is still active, but overall superfluidity seems to not be as popular these days. This shouldn't be surprising though, what is more amazing is that superfluidity managed to be popular for such a long, long time in the first place. $\endgroup$ – KF Gauss Aug 25 '20 at 23:01

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