More on Bubble Nucleation: Observational Experiment

Aim

Finally, a clue as to how steam bubbles form in boiling water!

Prior Knowledge

Description of the Experiment

From the previous videos, we had hypothesised that maybe there were impurities in the water around which steam bubbles were forming, but I did not know how to really test it. Maybe if one had pure water one could superheat it indefinitely and it would not boil? Then we spotted this piece of video footage.

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Questions

If you did not see it, watch the movie again. Notice that the source of the little stream of bubbles seems to move around. It seems reasonable to infer (although one cannot see it directly) that the source of the stream of bubbles must be some sort of microscopic impurity floating around in the water.

The story so far

So, there seem to be two sorts of ways in which bubbles form. The first is driven by contact with a superheated surface (Yes, we did measure the temperature and it was more than 100C at the bottom.) This is the preboiling. Basically it seems as though the bubble nucleates around a sharp edge or possibly some air trapped on the surface of the beaker. The bubble then expands until the buoyant force is sufficient to break the bonds with the surface and the bubble rises up and shrinks as it goes.

The second sort of boiling takes place around small impurities in the water. This may already be happening in the preboiling, but one will not see it because the small bubbles should disappear immediately. (This is only speculation). Certainly when all of the water is at the boiling point temperature, it becomes favourable for small bubbles to nucleate around impurities floating in the water. These bubbles then quickly expand, collecting more water vapour as they rise to the surface.

Questions

This whole analysis raises an interesting set of questions:

1. If one could remove all the impurities from the water and not perturb the beaker in any way, could one then keep on heating the water indefinitely past its natural boiling point without seeing it boil or change phase other than through surface evaporation? (I have to try this some time, but it requires equipment and money.) This idea seems ridiculous though. Surely water can only exist in liquid phase at standard pressure, below 100C? Are those phase diagrams lying, or at least not revealing the whole truth? I have no idea at the moment.
2. It should be difficult but not impossible to devise a model of bubble formation on surfaces. IE: Be able to predict exactly how big the average bubble should be before it can break off based on the surface tension properties of the heated liquid, the properties of the surface to which it is attached and the buoyant force. Then go and do an experiment boiling different liquids. Some time I will do this qualitatively to see first if there is any variation.

If anyone has ideas about answering these questions or has suggestions of more interesting questions to raise, please email us.