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Soap bubble foam made with helium floats up, but due to extreme fragility hardly counts as "material". There are many solid foam materials though - PUR foam, or styrofoam to name the most common. They typically use carbon dioxide for inflation though (usually produced from precursors of the foam, as a desirable side effect of their reaction).

But it shouldn't be too difficult to make solid foam filled with helium (or hydrogen) in proportions assuring positive buoyancy in air, and I can imagine desirability of it, at least as a filler in applications where mass costs a premium (transport, aviation) even if its structural properties were to be too poor for any other purpose.

Is such material produced? Is it used anywhere? Or if not, why?

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  • $\begingroup$ I suspect diffusion would soon make your foam lose all its helium. $\endgroup$ – Hobbes Jun 22 '17 at 12:48
  • $\begingroup$ You just need heavier air . en.wikipedia.org/wiki/Perfluorobutane $\endgroup$ – Carl Witthoft Jun 22 '17 at 14:34
  • $\begingroup$ This is a shopping question - Where can I find ____? What are your specs? What counts as a lighter than air material? You speculate that it should be possible to make a foam that lasts "months without losing buoyancy." Are you looking for a solid that is permanently lighter than air? Weather balloons can stay in the air for months. You could make a macro-foam by taping together a bunch of mylar balloons. What are your definitions of "solid"? $\endgroup$ – Chuck Jun 22 '17 at 19:07
  • $\begingroup$ @Chuck... Why do you hate me so? So many vile accusations about such an interesting question. $\endgroup$ – SF. Jun 22 '17 at 21:43
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No matter how good you seal it, when you inflate a balloon with Helium it will stay up for a while, but after a few days, it will lose its pressure.

Enough to realise that in solids, there is also a phenomenon of mass diffusivity, and therefore your foam will not retain the gas. This phenomenon is also called Permeation

Diffusivity in solids is very complex, and 'mostly' cannot be described with an equation as simple as with Fick's laws, and, in many cases, is not even isotropic. But there is still a condition that needs to be met to diffuse: the particle/atom you consider can place itself within the crystallography/pattern of your material. unfortunately for Helium, it is too small and will diffuse through all reasonable materials.

Diffusivity in solids is unfortunately making us unable to isolate a gaz but it is also positively used in a lot of fields. The most investigated is probably in microelectronics to make local implants of ions in semiconductors and therefore change locally its electrical properties.

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  • $\begingroup$ This is an incomplete answer. For example, an H2 balloon will not suffer as great a diffusion rate as He. $\endgroup$ – Carl Witthoft Jun 22 '17 at 14:35
  • $\begingroup$ Additionally, in closed-cell foam, the sheer number of membranes to be crossed on the way out should be large enough to reduce the diffusion rate considerably. Add a good coating on the surface, and this should last at least months before losing most of buoyancy. $\endgroup$ – SF. Jun 22 '17 at 14:43
  • $\begingroup$ @CarlWitthoft at what point do I imply that the $H_2$ has a better or worse diffusion than $He$? As mentioned diffusion in solids is very complex, most of the time we only use experimental or partly experimental models. The only thing I can say is: if the necessary condition is met there is diffusion. $\endgroup$ – Jonathan Jun 23 '17 at 9:37
  • $\begingroup$ @SF. your logic is overall interesting,If you ever manage to find a process to create such a foam, I would be interested to hear the weight variations over time. And if you allow me, I am not an expert of foams, but I think a lot a of foam are produced from a reaction that release non $He$ gazes. $\endgroup$ – Jonathan Jun 23 '17 at 10:10
  • $\begingroup$ @Jonathan: Yes, the foam would likely need to be aerated "mechanically". and certainly that wouldn't be easy, but I'm inclined to believe even if the material loses its properties over time, it could still be worthwhile in some applications. $\endgroup$ – SF. Jun 23 '17 at 10:27
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Here is a link to an aerogel 7 times lighter than air.

They are mainly in experimental phase. To make them into practice, they should be cheaper as the equivalent system filled with Helium.

They are mainly porous materials derived from a gel, from which the liquid component has been removed.

The result is like this (it is a different aerogel as in the first link):

enter image description here

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  • $\begingroup$ ...so why doesn't it float?? $\endgroup$ – SF. Jun 25 '17 at 0:53
  • $\begingroup$ @SF. It is lighter as in the first link. $\endgroup$ – peterh - Reinstate Monica Jun 25 '17 at 5:50
  • $\begingroup$ I voted down sorry, the 'lighter then air' is too confusing. It has been created just to hook the readers on a commercial website. I can change my mind if updated more scientifically. $\endgroup$ – Jonathan Jun 25 '17 at 20:34
  • $\begingroup$ It is heavier than air. That website is crazy. $\endgroup$ – Millemila May 8 '18 at 21:09
  • $\begingroup$ @SF. Maybe air diffuses into it, increasing its density. Its "raw" density (i.e. if it wouldn't have holes) would be probably like any plastic. $\endgroup$ – peterh - Reinstate Monica May 8 '18 at 21:16
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There is a material type named aerogel and you can find some information on this quora topic.

link

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    $\begingroup$ Link-only answers are discouraged because of the risk of stale links. SInce your linked page refers back to the Wikipedia page, I recommend linking there and additionally quoting pertinent sections here. $\endgroup$ – Carl Witthoft Jun 22 '17 at 18:18
  • $\begingroup$ Aerogel is very light but still heavier than air, and AFAIK it's an open-pore foam so it wouldn't retain a lighter-than-air gas. $\endgroup$ – SF. Jun 22 '17 at 21:44

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