why does food colouring act in this way?

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paul.fr

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why does food colouring act in this way?
« on: 16/03/2009 14:42:37 »
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?

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paul.fr

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why does food colouring act in this way?
« Reply #1 on: 16/03/2009 14:57:10 »
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

Quote
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?

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paul.fr

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why does food colouring act in this way?
« Reply #2 on: 18/03/2009 19:27:01 »
Anyone?

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Offline LeeE

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why does food colouring act in this way?
« Reply #3 on: 20/03/2009 12:00:25 »
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.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!