Physics Asked on August 5, 2020
Discussions in comments below How can aerogel be lighter than air? really make an answer to this important. The conclusions there are that aerogel could in some cases be "lighter than air" if one takes only the mass of the aerogel material and ignores the mass of any trapped gas within each cell, and divides by the volume of the whole thing (aerogel including all trapped volume in its cells).
For #2 the latter is more interesting because it suggests that aerogel could conceivably be produced that could float away even at STP.
From NASA's article Aerogels: Thinner, Lighter, Stronger:
Since their invention, aerogels have primarily been made of silica. The silica is combined with a solvent to create a gel. This gel is then subjected to supercritical fluid extraction. This supercritical fluid extraction involves introducing liquid carbon dioxide into the gel. The liquid carbon dioxide surpasses its super critical point, where it can be either a gas or a liquid, and then is vented out. This exchange is performed multiple times to ensure that all liquids are removed from the gel. The resulting material is aerogel.
Removing all the liquid out of the gel implies that all the pockets made of silica that were filled with that liquid have to be interconnected, so the air in an aerogel can move from one pocket to another and eventually out of the aerogel !
So the trapped gas in your questions is not that "trapped" after all.
Answered by Conelisinspace on August 5, 2020
To my knowledge the manufacturing process requires evaporation of solvent; when all of that solvent is gone the remainder is the solid that comprises the aerogel.
To my understanding, it is as if under water a scaffolding structure is assembled out of parts that are neutrally buoyant. The neutral buoyancy allows for all the parts to come together simultaneously. (Of course, to end up with the desired properties the parts must end up in a very sparse structure.) When the parts making up the structure are bonded to each other firmly you can allow all of the water to flow away.
That is, my understanding is that in order to be able to fabricate the aerogel at all the structure must not trap anything.
Answered by Cleonis on August 5, 2020
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