[paul.fr]

If you have a still glass of water and gently pop a drop of food colouring in to it, it forms a circle that appears to be spinning. As the drop falls mid-way down it slows down forms (what lloks like) a crown type of patter, with smaller rings on the tips. Why does it act this way?

[paul.fr]

A quick search and it would seem that RD has answered the first part of the question here:
http://www.thenakedscientists.com/forum/index.php?topic=21146.msg236430#msg236430

The ring is a toroidal vortex (like a smoke ring): a doughnut of water is rotating around the air, trapping it.

The force created by the momentum of the rotating doughnut of water surrounding the air ring is greater than the buoyancy force of the air, so the air ring does not rise.

But does anyone know the answer to the second part? Why does the ring collapse, and why does it collapse in to that specific shape / pattern?

[paul.fr]

Anyone?

[LeeE]

I don't think there's a simple answer Paul.  You'd have to do some serious CFD to find out exactly what's happening i.e. how the turbulence behaves in the volume of water.