@_thegeoff who specifically might find it interesting
Consider a pot with a convex bottom (i.e. it will stand on its midpoint) with sufficiently large radius of curvature to make it stable when filled to any level. When you put such a pot on an electric hotplate and get it boiling, it starts rocking: https://www.youtube.com/watch?v=vB_szsLa3zk
I have a hypothesis on what's going on (the pot rocks away from the area where the boiling is more intense due to density difference, contact with hotplate increases rate of heating, so there's more heating on the side that's currently lower and thus any rocking gets amplified). Sadly, I don't have the pot anymore and didn't thing at that time of any experiments that could falsify this hypothesis.
@robryk (1/2) Lovely! My hypothesis here is something most people don't realise about boiling water with a hot surface:
Turn on an electric kettle. It starts to make noise as the water gets hotter, and gets louder the hotter it gets, but then just before it boils it gets quieter again.
Seriously, I made a graph a few years ago.
@robryk The critical thing is what makes the sound: it's not bubbles going pop (well, it is at the very beginning as the water expels dissolved gas), it's bubbles of gaseous water...not "steam" as in stuff in your bathroom, or mist, but actual gaseous H2O.
This forms close to the hot surface, a pocket of it starts to rise. But it quickly hits cooler water and condenses, and that happens *fast*!
The collapse is violent, a little hammer blow. It's what makes the noise, and maybe moves the pan?
@robryk The reason the kettle gets quiet again is that close to a full boil those pockets of gaseous water can get all the way to the surface, because the water is hot enough to allow that, and they go "sploosh" in the air, not collapsing *BANG* inside the fluid.
Question is, does the pan rock less at a full boil than near it? Cos it could just be a roiling fluid kicking the centre of mass around.
But violent bubble collapses are too cool to not consider 😉
I don't think it rocked less, but we didn't really test it for too long, because unless you held it it would splash its contents out at a surprisingly high throughput.
I really should find a pot to break. :)
@_thegeoff
Ah, and I think the rocking would start before major bubble-implosion-noise and clearly visible bubbles (and I messed up the explanation I was thinking of at that time -- the density changes would come from thermal expansion as opposed to steam creation).