Question: It seems I've been seeing a lot of uses for foam on this site, from a weapon to a spaceship defense mechanism. However, the details on what the foam is made of and how it works always seem lacking to me; thus, I worry that many of these foams are just science fiction. 

My question is **is there a foam that can be used to quickly construct airtight walls solid enough to keep humans out?** I imagine using this foam on a spaceship to trap the enemy, or alternately to fix hull breaches. Perhaps there is a fast-acting variant that can be used as a grenade to immobilize enemies. As it is a foam, it could be carried around much more easily than other wall-building materials, thus I could outfit my space marines with a few cans apiece. 

So, does this foaming material exist, or is there a theoritcal way to produce it? If so, how big would the initial substance be, and how long would it take to dry/harden? 

The foam need not be activated by air; on the contrary, it would be great if it contains its own reactants, and can be activated in a vacuum. 

It need not last; it should only be functional for a minimum of a couple hours. If it breaks down after that, that's fine. 

I would like it to be as strong as possible. If it can get to concrete levels, that would be best, but as long as it's very difficult for a human to burrow through that'll work.

Answer: 
Foam is, by definition, simply a lot of bubbles.

**Bubbles don't work in a vacuum.** (Theoretically they do, but their behavior isn't interesting since they explode — emphasis on *explode* — immediately.)

So this gives you a few upsides and a few downsides. I could see a viable 'cement-like' foam that hardens *because* the water or other liquid with which it is inflated immediately evacuates and vaporizes in outer space (e.g. when repairing the wall of a spacecraft) leaving only the igneous-rock-like matrix behind.

@AndyD273 points out quite correctly that foam in a vacuum expands far more than it does in atmospheric pressure. But remember that this also means that you have far thinner bubbles in much more space, leaving you with a much weaker structure. 

Your wall material would have to be pretty (1) flexible and (2) strong to begin with, even before spraying.

**But this gets more complicated when working in atmospheric pressure.** You're limited at the very *fastest* by the entropy-fueled diffusion and expansion of gas. This leads me to believe that a foam used as a quick-spray detainment weapon would need to have *highly* reactive gaseous compounds that are stored separately and then injected together into the foam as it's released, causing that sort of high-speed expansion you're looking for. (Side-benefit: **Such a quick expansion would either release or absorb a lot of heat**, depending on the mechanism. I could viably see your foam curing due to its own high heat, or freezing due to its below-zero temperatures.)

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Side-note: Anyone who's tried to use their hands to tear that foam insulation stuff knows from experience that, even though it's *possible*, it's an enormous pain to actually do. Easy to dent it, not easy to rip.

**[EDIT]**: As @JDługosz points out, a foam in a vacuum can cure from the outside in... But that would lead to a pressurized interior. And [we all know what happens](https://www.youtube.com/watch?v=6V2eCFsDkK0) when brittle materials harden on the outside and leave a still-liquid, still-cooling interior...
The only foam I have much experience with is the expanding-foam insulation that comes in a spray can. From that, I find it fairly believable that you could plug a hole in the side of a ship with enough of it, though I'd imagine it would also unavoidably plug up a good deal of the interior of the ship. I've also unfortunately never been able to test it in vacuum.

The hard part is making it durable enough to prevent tunneling. But if the human-to-be-trapped lacks access to digging tools, you might be able to keep them in by embedding hand-lacerating caltrops of some sort in the foam mixture. (Maybe these caltrops "grow" there through rapid crystallization?) There's not much point in digging a tunnel out of prison if you bleed out halfway through.

Actually, if you use enough foam, and/or if it sets quickly enough, you could just trap the intruder like an insect in amber. That would also suffocate them almost immediately, unless they had a suit with an independent air supply, in which case you'd just have to wait until that supply runs out.