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Something a little different to our usual fare. I was boiling a pan of water for cookery the other day, and got to wondering what caused the location of the bubble streams from the bottom of the pan. The stream of bubbles seems to come from the same spot of the pan, is this a quirk of the fluid flow? Imperfections in the pan? These streams of bubbles appear to continue for several minutes until the water fully boils and it becomes too turbulent to follow anything.

Any thoughts?

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At least on my burner which is electric, the bubbles form in a spiral in the same way my electric burner applies heat in a spiral. I've always assumed that the bubbles form is specific spots because of non-uniform heating. The thickness of the metal in the pan and how the heat is applied are probably the biggest factors. Also, bubbles don't want to form near the sides because a lot of heat is lost through the sides. –  Brandon Enright Mar 30 '13 at 20:15
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I think the OP is also asking about the specific points that seed bubble formation, not just the general area of the pan. Bubbles come from hot regions, yes, but also only from very specific points within those regions. –  Chris White Mar 30 '13 at 21:10
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More bubbles form at aspersions/surface roughness? –  drN Mar 30 '13 at 22:48
    
Just a comment, this seems abstractly related to formation of structures in the universe, which is caused by lack of symmetry. So if the water, pressure, heat transfer etc. had perfect symmetry, the entire water should just become a bubble at the same time. –  raindrop Mar 31 '13 at 0:08

1 Answer 1

up vote 4 down vote accepted

According to Wikipedia

"For a vapor bubble to expand, the temperature must be high enough that the vapor pressure exceeds the ambient pressure (the atmospheric pressure, primarily)"

At Cornell they did a study of Critical Droplets and Nucleation, and they observed that surface tension is what allows this vapor pressure change and causes the bubbles to form. This can be caused by imperfections in the pan due to heating of the pan itself, scratches in the pan, and other added areas of nucleation such as salt or sand. Not due to impurities of the water, although different impurities do heat at different rates than water.

(Very Dangerous) If you heat distilled water in a smooth container (with no nucleation points or imperfections) in the microwave it exceeds the boiling point because no bubbles are allowed to form. At the point you shake the cup or add something that breaks the surface tension you invoke a vapor pressure change allowing boiling to take place, rather violently because you've created new points of nucleation.

When you heat metals (welding and cutting included) you create something called a heat affected zone (HAZ) where the microstructure of the material and the properties of the material itself are altered by the heating process. In your pan you're causing different materials in the metal itself to shift (based on the heating element placement) which is what's helping to create [some of the] nucleation sites.

The shape of the pan, imperfections in the pan, and the surface tension caused by the bubbles and the lack of bubbles on the surface, and the pressures in the vertical flow column allow the bubbles to form uniformly, creating streams of rising bubbles.

Stuff in the pan like salt, rice, and noodles stuck to the bottom of the pan can help to create nucleation sites as well.

This would be my best guess.

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