Whenever I put an ice-cube into a glass of hot water, so that it melts quickly, and it is initially rotating slowly, I noticed that its rotational speed increases as it melts and 'shrinks'. Why?
I thought of conservation of angular momentum, but does that really apply here? The part of the ice that melts into water would be 'carrying' off some of the ice-cube's angular momentum as it breaks away and diffuses into the surrounding water. So I see no reason for the remaining ice-cube to rotate faster.
I can only think of one possible reason, and it is just a guess: The friction between the water and ice causes the melting part to form eddies that spin in the opposite direction to the ice-cube, and so the ice-cube loses much less momentum in melting since its boundary is actually moving together with the adjacent eddy boundaries. If the eddies expand as the ice-cube shrinks, it seems we would have to conclude that the ice-cube must rotate faster to preserve total angular momentum.
But such an analysis seems very weird, and cannot be a proper picture of what is really happening since the 'number' of eddies must decrease as the ice-cube shrinks, so they must combine in some manner, and I have no idea how any of this works out.
So, what really is spinning the ice?