How to make an ink ring in a glass of water, in this Moment of Science.
You need a tall glass of water—the taller the better—a bottle of ink, and an eyedropper.
After you fill the glass with water, put it on a steady table and leave it alone for a few minutes, to allow all the turbulence in the water to settle down. Now take some ink in the eyedropper and release one drop from a height of one inch above the center of the water.
The ink will form an expanding ring, descending through the water like a smoke ring traveling through the air.
The ring keeps its shape because of the way water moves in it. You can get the idea by thinking of a rubber O-ring, maybe something like the rubber rings that are used as drive belts in upright vacuum cleaners. You can twist one of those rings inside out without changing its overall circular shape.
What's happening with the water is something like that. The ink ring that you see is actually turning itself inside out as it travels to the bottom of the glass. The turning motion was started by the ink drop falling into the water.
The ring stays sharp and clear as it descends. That shows that water and ink in the ring don't mix very much with the rest of the water in the glass, at least not at first. There is some friction between the water in the ring and the surrounding water, and after traveling a long way, the ring will start to get fuzzy around the edges. But within the dimensions of a drinking glass, the ink ring will stay amazingly sharp and clear all the way to the bottom.
To see the ring, fill a tall glass with water, let the water become quiet, then release one drop of ink from a height of one inch above the center of the water. You'll see a beautiful expanding ring descend to the bottom of the glass.