I've heard from others that air is what causes sealant to seal. People have said they've read that "somewhere" but I never seem to be able to find those articles.
The reason I think air can't be the sole factor is that there is air inside the tires—in mountain bike tires there is a lot of it. Why wouldn't it just coagulate there? And if you say, well, over time it dries out, that's true—but a puncture doesn't take months to seal.
On top of that, I've accidentally left a bottle of orange sealant with the cap off literally for months, and there was only the barest trace of "rubbery" sealant on the threads of the cap neck.
I imagine this must have something to do with either
- the cold produced by the expanding gas coming out of a puncture, or
- the low pressure caused by the expanding gas moving rapidly (see Bernoulli's Principle)
- some other unexplained factor.
The reason I want to know is because if I knew the principle behind sealant sealing I might be able to figure out a way to prevent undetectable slow leaks, which I've been getting off and on since I switched to tubeless on my road bike.