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A few days ago, when boiling potatoes I only cut each of them in half (leaving a flat face in each half). When the water started bubbling a little (and also later when it was boiling) the flat face of each half potato was facing the bottom of the stewpot. I noticed that if you push the potato against the bottom surface the water starts to bubble quite a lot even if the water is not that hot.

I observed that the more suface area (of the flat face), the more intense the bubbling. Other than that, I haven't experimented that much: temperature of the water, surface area of the flat part, overall volume (or height) of the half potato, size of the stewpot, temperature of the air around, etc.

Does someone have an expalanation of this phenomenon and/or any prediction of what will happen if I change any of the factors?

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    – rob ♦
    Commented Dec 26, 2019 at 19:24

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I noticed that if you push the potato against the bottom surface the water starts to bubble quite a lot even if the water is not that hot.

Pressing the flat faces of the half-potatoes against the bottom of the pan causes the thickness of the water layer (between potato and pan bottom) to be reduced strongly.

This in turns makes the thinned water layer heat up quickly (compared to the surroundings) and thus reaching boiling point fast, which manifests itself in localised more vigorous boiling.

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