Ahhh. The science that is boiling. No kidding. I remember a lecturer of thermodynamics (very trendy young German guy) going all glassy-eyed and excited as he described the change of phase of water. He tried to tell us that it was very exciting. (It was about that time that I deciced I didn’t really want to be an engineer. They live in such little worlds. But that is another story.)
Let’s back-track a little. You know about surface tension. Sure you do. That’s what enables some insects to walk on water. It is also why little droplets form on the windscreen of your car instead of a continuous sheet of water. And critically, it is what gives water droplets their shape. The forces holding the molecules together at the surface tend to pull tight to form the smallest possible area. Ok. Now imagine a bubble. The “skin” of the water surface curves the other way. That’s ok, but in this case the forces pull tight to reduce the size of the bubble so that they are as small as possible. When the radius of the bubble is very small (microscopic) these forses are great enough to completely collapse the bubble so that the steam inside goes back to liquid.
So, for any boiling liquid there is a critical radius for bubbles. Any bubbles smaller are instable and will shrink and disappear. Any bubbles that are larger are able to grow.
Which then leads to the question, how do you get bubbles in the first place? If the small ones shrink, they never get a chance to grow into big ones. The answer is that the majority of bubbles form at the surface of something. They start as a little lens shaped thing on a flat surface. That means that their radius is not so small, they don’t consume themselves by their own surface tension and they are free to grow. Typically the flat surface that bubbles form on is the sides of the pot or element or something. Just watch the first bubbles form on the immersion element, grow, rise to the surface and another form at the same place. (This is what got my lecturer all excited. Me, I’d prefer to watch a movie.)
Now. Take the case of milk. It is a mixture with very fine solid particles suspended in it. This means lots of wonderful nucleation sites where a bubble can form. When the milk gets hot enough and bubbles start to form, lots and lots of very small bubbles can form all at once throughout the mixture. There are so many nucleation sites and so many bubbles that they do not individually get very big. Also, very small bubbles do not rise to the surface as quickly as big ones. So, you get a very fine foam that expands in the pot instead of a few smaller bubbles that expand, rise and break through the surface.
Which is why you should never turn you back on boiling milk.