I just

looked up the word "scum". Do You really mean

that? I would say what You see is fog, tiny water droplets

condensing at the upper edge of the laminar layer

of steam on the tea.

Georg

The layer on top of the tea is calcium carbonate (about 15%) and complex organic compounds from the tea. The actual percentage of calcium carbonate will depend upon the hardness of the water that has been used. In very strong tea, the acids will react with the calcium carbonate to produce carbon dioxide. This will cause the layer to thin out or may even remove it. Adding lemon to the tea will have the same effect.

The tea in contact with the cup will cool faster than the tea in the middle and this will result in convection currents. The flow of tea at the surface, from the middle of the cup to the walls, puts the thin layer under stress and causes the cracking. The thickness of the layer is unlikely to be uniform and the cooling of the cup is also unlikely to be uniform because of air currents and variations in the cup thickness. This gives rise to a chaotic behaviour. The cracks and the variable thickness of the layer on top of the tea will also have some influence on surface cooling of the tea and further affect the formation of the cracks.

You are correct in thinkeing that you are looking at a chaotic system. The fine detil in a chaotic system cannot be predicted.

What video?

I think you would have to host it or put it on youtube and give us the link.

... not sure where the scum comes from, but if it spoils your enjoyment of a nice cuppa tea, scoop it off the surface with kitchen paper.

There are two factors at work on the top of your cup of tea: a surface "scum" described earlier by a respondent as calcium carbonate; and the turbulent eddies refreshing the surface.

The turbulent eddies are setup by convection currents in the hot tea. These eddies disturb the surface and crack the scum much like the sea causing cracks across ice sheets. The eddies disturbing and refreshing the surface can be described by the Kolmogorov Turbulence Theory (see http://en.wikipedia.org/wiki/Turbulence).

This is where my memory ends though... I remember this as being one of the last classes at university and then later, at the graduation luncheon, watching eddies refreshing the surface of my coffee thinking "Kolmogorov Theory!".

Kevin.