By the way, here's a neat video that demonstrates an important difference between laminar flow and turbulent flow.
[YOUTUBE]http://www.youtube.com/watch?v=X4zd4Qpsbs8[/YOUTUBE]
The reason it works is this: the fluid into which the dye is placed is very viscous. So long as you stir it
slowly, flow is laminar and all the molecules are moving in the same direction. So, if you're careful, you can reverse the direction of stirring and get your original drops of dye back.
This works because by stirring in reverse, you're reversing their direction of movement and returning the molecules to where they started. So long as you stir
slowly and in a precisely reverse pattern to the original stirring that dispersed the dye throughout the fluid, fluid flow remains laminar and so you can reverse the process by which you dispersed the dye through the viscous fluid and get your dye drops back.
We used to do this trick in a Biomechanics class I took; we'd put drops of dye into corn syrup, stir it up to disperse it, then reverse the direction of stirring to get the drops of dye back. It was always a fun trick, and it really impresses people who aren't expecting that sort of thing. (You can win money off people by betting them you can "unstir" a mixture.)
The trick, though, is to use a very viscous fluid and not to stir too fast. Otherwise, fluid flow becomes turbulent and the molecules are
not all moving in the same direction. You
cannot unstir the mixture if flow becomes turbulent.
Cheers,
Michael