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When boiling water, I've noticed that bubbles will appear to grow at the bottom of the glass for a period of time and then rise. I've read that bubbles form from nucleation centers but I don't know the specifics of the nucleation dynamics. Is there a possible scenario where the bubbles, while still appearing to adhere to the bottom of the glass, are in an equilibrium where the bottom par of the bubble is being formed by water evaporating into gas, but at the top of the bubble, which might be slightly cooler, the gas is redissolving into liquid phase? If the two rates are the same then the bubble would appear stationary. At some point the water heats up enough to have rate of evaporation greater than dissolving throughout the entire bulk of the liquid and the bubble will survive its transit to the top of the liquid. I think that falling drops experience a dynamic that is th opposite of this.

A couple other thoughts that I've considered:

  1. I know that boiling occurs when the vapor pressure of water is greater than atmospheric pressure. I believe that this condition is achieved even when the bubble is stationary since the bubble is displacing the surrounding bulk water and air.

  2. I think that carbonated beverages can have bubbles stick to the side of a glass, but I suspect that the reason for this is different but i don't have an intuition about why carbon dioxide bubbles in a cold drink might stick, since the beverage is cold.

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The bubbles are already on the surface, they are just too small to see with the naked eye.

Wetting a surface, even at room temperature, results in tiny gas/vapor bubbles at defect sites due to surface tension. For example, surface tension prevents water from seeping into tiny crevices (on the order of microns).

These tiny gas pockets expand when heated, and eventually you can see them. They were on the surface the entire time, they just expanded. They stay on the surface because surface tension pulls down and balances the upward buoyant force.

If you keep adding more energy, however, the gas in the bubble will expand. Eventually the bubble will eject from the surface because the surface tension scales inversely with bubble radius, so the force holding it back decreases. Furthermore, as the bubble increases in volume at the surface, it gains an appreciable buoyant force that overcomes surface tension. At this point, the bubble rises.

You can actually superheat water above the boiling point if you have a surface that has small enough defects, since this makes it more difficult for gas bubbles to be trapped when the surface is wetted.

Anyways, the bubbles seem to stick to the sides of the container because they were always there to begin with, thanks to surface tension. You only see them when higher temperatures cause the gas inside them to expand.

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  • $\begingroup$ Good job drew. are a physical chemist? $\endgroup$ – niels nielsen Dec 22 '18 at 7:02
  • $\begingroup$ Thanks @niels nielsen. Nah, I'm a mechanical engineer $\endgroup$ – Drew Dec 22 '18 at 9:35
  • $\begingroup$ I'm not sure if this answer is correct about what it addresses, but I don't think it addresses the formation of bubbles containing water vapor after any air initially present has already escaped. $\endgroup$ – R.. Dec 22 '18 at 17:02
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    $\begingroup$ @R.., I did not explicitly explain how new bubbles form when old ones are ejected, but the reason they form is the same as why they form when the surface is initially wetted. When a bubble lifts off, colder fluid rushes in to take its place. The defect site is still there, however, and the new fluid fails to fully wet the surface again due to surface tension. Thus, a new tiny vapor bubble is formed and the process repeats. The presence of defect sites, along with surface tension, causes vapor bubble formation (even in the absence of boiling). $\endgroup$ – Drew Dec 22 '18 at 17:37
  • $\begingroup$ I think I understand now that at the microscopic level, surface tension prevents the bulk water from completely penetrating the defect on the adsorbing surface. The surface tension also prevents the water from creeping under the bubble. Does this mean that there is initially no buoyant force upwards, but as bubble swells, there is a slanted region of bubble (not perpendicular) that provides a surface for water to push up? Thanks $\endgroup$ – lamplamp Dec 22 '18 at 19:27
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It is expensive, in energy terms to, make a surface like the surface of a bubble.

By sticking to the walls or bottom of a container some of the surface is free, so it is energeticly favorable to stay partly attached until enough gas forms that the ratio of surface area to volume of gas reaches some limit (which depends on surface tension and the nature of the liquid and surface)

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