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Old Aug 27th 2017, 10:15 AM   #1
Krampf
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Strange fun with food coloring questions

When opening an old container of food coloring, a bit of the dried coloring dropped into some water, and immediately began zipping around the surface. I recreated this by putting several drops of food coloring on a piece of plastic wrap, and putting it in the sun to dry. Flexing the plastic gives you nice pieces of dried coloring, which zip wonderfully. I am guessing that the motion is because the coloring dissolves faster at its sharpest point (more surface area), and that it is disrupting the surface tension, which lets the surface tension in front pull it along.

The one flaw that I find in this explanation is that the coloring will often cross its own trail, and you can keep dropping more and more pieces into the same water, and they keep zipping around. Any thoughts on why this happens would be very welcome.

If you try this yourself, you may notice something else strange. If you use red food coloring, when it dries it will appear green in reflected light. If the light comes through the dried coloring, it still looks red, but when lit from above, it looks green, even under a microscope. This seems to be structural, as powdering it causes it to look red again.

If you dry green or blue coloring, they look red in reflected light, but keep their own color for transmitted light. Any thoughts on this would be very welcome too.

For clarity, the red food coloring contains: WATER, PROPYLENE GLYCOL, FD&C REDS 40 AND 3, AND 0.1% PROPYLPARABEN (PRESERVATIVE).

The green contains: WATER, PROPYLENE GLYCOL, FD&C YELLOW 5, FD&C BLUE 1, AND PROPYLPARABEN (PRESERVATIVE).

I play with things like this frequently to develop activities for teachers, but I want to be sure that I am explaining it properly.

Thanks for any help.
Robert Krampf
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Old Aug 28th 2017, 11:03 AM   #2
Krampf
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Update

I now strongly suspect that surface tension is responsible for the "zipping" around of the coloring. I put a tiny bit of dish washing detergent into the water first, to disrupt the surface tension. When I dropped the bits of color, the pieces just sat there on the surface, dissolving without moving.

Still puzzled about the color change in reflect light.
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