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We have a glass jug water kettle with a flat heat plate as the bottom. It boils water quite vigorously in 2-6 minutes depending on how full with a 1000+W element.

Just today I noticed that very shortly (under 30 seconds) after having switched the full kettle on there were bubbles on the heating base that were likely there from when I filled the kettle.

After about a minute of heating usually (fresh or additional) bubbles of trapped gasses form and some rise and some float to the top.

My question arises from as on this occasion I had jostled the jug when switching in on and the water was still oscillating to and fro and I saw the small (0.5mm to 1mm diameter) bubbles jiggling side-to-side in time with the wave action. This was all normal except the bubbles did not detach from the bottom even though they were free to do so and if I had wanted to free some stuck bubbles I would have tried to do it by shaking the jug (or tapping it).

The water was nowhere near boiling, under 20 degC certainly. The element had started to warm up as I could just start to see the diffraction effects from the density changes in the warming water. I think the bubbles were a result of the water being filled from the tap rather than the bubble of trapped gas that rising more normally.

Does anyone have a clue what keeps the bubbles stuck to the bottom in what looks like no real contact (an inverse Leidenfrost Effect underwater) but still unable to rise from the bubble buoyancy?

EDIT:
The mobile bubbles were around the outer part of the base where the most heating occurs. The stuck bubbles in the middle were not moving.

I did the experiment again and can say that the bubbles are from the dissolved gasses as they formed just shortly after the heating was started, the size and amount of bubbles from filling was MUCH less.

If the bubbles grew large enough they would break free and float. After the water was hotter and the bubbles were not enlarging much they did not seem move as freely.

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the bubbles adhere against buoyancy forces because of surface tension effects. In water, surface tension declines as the temperature rises, so once the water gets hot enough the bubbles start to detach and float to the surface.

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  • $\begingroup$ I suspect something like that but what surface is active when the bubble is free to move almost as if it was no longer in contact. Is this why the bubble in the heating zone were more free to move and those in the middle were more firmly attached? $\endgroup$ – KalleMP Nov 22 '18 at 18:52
  • $\begingroup$ without seeing the kettle bottom I cannot tell you. $\endgroup$ – niels nielsen Nov 22 '18 at 18:56

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