Why water needs rugged surfaces for bubbles to form while heated, and why air impurities are necessary to form vapour



Something has been bugging me ever since I learnt about superheated water in microwaves.

People keep saying that it’s because the surface is too smooth and air bubbles can’t form. But I can’t seem to find an easy to digest information about WHY smoothness prevents bubbles from forming.

I found a video explaining that heating water causes the hydrogen bonds to breaks, which makes the water molecules move about and that’s what causes bubbles. But this explanation doesn’t explain why a rugged surface is necessary in the process :/


And I also saw that impurities in the air are necessary for water vapour to form, and that’s how clouds are formed around micro-particles from deserts. How does that work ?

Thanks 😀

In: 1

This isn’t exactly what’s happening, but it’s analogous:

Take an intact piece of printer paper. If you just grab each side in one hand and pull, you have to pull really hard before the paper will rip into two pieces, because your energy is being spread out across the entire sheet. If the paper had even a small rip on the top or bottom edge, then tearing it would be super easy because all the force gets transferred into that one spot. You don’t have to pull very hard to make the tear wider and wider until the entire page rips.

Similarly, if you have superheated water in a smooth container, the water is ready to boil, but it’s missing a trigger to set it all off. It’s easier to join an existing bubble than it is to create your own, so every water molecule is waiting for one of its buddies to be the first one to start. It’s as if the water is trying to boil across the entire surface of the container at once, and it’s like trying to pull apart a sheet of paper that doesn’t have a tear in it. The energy is being spread out evenly enough that no part reaches the tipping point. But as soon as one region gives in, the whole thing goes really fast.

It’s called nucleation. Simply put, things don’t tend to change phase easily unless you give them a helping hand.

The rough surface allows the molecules of the fluid to organise themselves in a different way, facilitating the phase change.

If you want to understand it more completely it’s worth a google because the mechanics of it are probably quite complex.

Be ause starting to form a bubble or rain drop takes a lot of energy. The rugged surface or the air impurity gets the process started to get to a point where it takes less energy for the bubble or rain drop to form.

Imagine a small bubble trying to form. It’s a sphere, but it’s so small that the surface tension around it outside is very high because of how tight it has to curve around the bubble. This potential energy needs to be collected before the bubble can form, so ots very unlikely to happen on its own. If it does happen, it’s much easier for the bubble to shrink and dissappear than it is for the bubble to grow to a stable size. Once the bubble reaches a certain size, it actually takes less energy to grow than it does to shrink, so at that point, it’s stable. What the rugged surface does is it offers the gas a place where it doesn’t need to create as much surface tension to form a bubble, so it can get past that small, unstable phase faster and with less energy.

It’s the same idea with rain drops. Literally the same concept.

Also, it doesn’t need it to form vapor, just droplets. Droplets are just an aeroslized liquid, vapor is just a gas.