@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 😉

@_thegeoff

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).

@_thegeoff

But why would the liquid start roiling in the first place? Liquid surface tilt and pot tilt are obviously coupled resonators, so practically you will get both oscillating if you get either.

@robryk convection currents love to be conplex. Heat a couple of inches of oil in a small pan and you'll see cool geometric diffraction patterns forming and shifting. Now speed up the idea and add turbulence from boiling!
Boiling water is far from a simple problem, despite being one of humanity's earliest technologies!

@_thegeoff

But they would impart only angular momentum to the pot, and not momentum (because they leave the surface level roughly constant and unchanged if averaged over horizontal circles a cm or two in radius). So, this explanation could be tested by letting the pot slide ~frictionlessly sideways and seeing whether its center of mass moves sideways, right?

@_thegeoff

Yeah, you can see how many assumptions one must make to have students model convection: https://ipho.olimpicos.net/pdf/IPhO_2008_Q3.pdf (for example, the convection cell size is clearly mostly a property of the fluid, and yet I've never found any intuitive explanation for what the size should be or even any Reynolds-number-like symmetries).

Hmm... somewhat relatedly: can one get a conservation law from the symmetry that gives the Reynolds number (via Noether theorem on presumably some weird space that includes fluid properties)?

@robryk If I could apply Noether to turbulent flow I'd be in a very different job 🤣

@_thegeoff

That is a reasonable alternative hypothesis (that we basically get white-ish noise that pushes a dampened pendulum around, so the pendulum will end up with some amplitude of resonant oscillations). It should be easily verifiable if I had such a pot (basically insert a membraneless speaker with the axis oriented radially, play white noise, and see what happens). Perhaps I should find an old pot and reshape it that way with some wood and a hammer.

I realized that I'm actually not sure how a harmonic oscillator driven by noise from e.g. a Wiener process (but without damping) should behave (would its amplitude diverge? probably not).