Two bubbles

[nancylebov]

If there were an infinite universe full of water, and there were two bubbles of air in it, would they move towards each other? Away from each other? Stay put?

If they moved close enough together to touch, would they coalesce into one bubble?

Would anything turn out differently if the universe were full of cold honey?

I don't have the answer--a couple of my friends who know more physics than I do kicked the initial question around. They and I have intuited the same answer, but no one has a proof.

Comments

[juuro.livejournal.com]

Cold honey -- same end result, slower process.

On touch, definitely coalesce. The total energy of one big bubble is lower than that of two smaller bubbles.

Initially, the bubbles would be drifting rather randomly. It is conceivable that when they drift "sufficiently close", they would be starting to "shade" each other from the random buffeting of the surrounding fluid, and be pressed closer together. What this "sufficiently close" means, I have no clue.

[brooksmoses]

On touch, not definitely coalesce; you've got to break the surface tension in the wall between them first. That's actually pretty hard to do, and in a universe without impurities it will be even harder. Most bubbles in our universe tend to simply bounce off each other.

(Soap bubbles are really bad for the intuition on this, because -- being film bubbles rather than bubbles in liquid -- they act quite differently. To begin with, they stick to each other and happily produce large flat regions of contact surface that are easy to break. Bubbles in liquid don't do that, unless they're physically pushed into each other.)