Popular Science has a great article about the creation of coloured bubbles. You’d think it was a pretty basic thing to do, but trying to get dyes to stay dissolved in the thin layer of surfacant and water that makes up a bubble, while at the same time making sure that the dye is not toxic or staining, is a pretty difficult task.
The last couple of pages of the article describe the solution in detail, and it’s certainly not the solution that you’d naturally come up with. Rather than using a washable dye, the creators ended up with a dye that totally vanishes when dry. Pretty amazing really, and as the article mentions, there are hundreds of possibly applications for such a dye:
Among the ideas Kehoe has already mocked up are a finger paint that fades from every surface except a special paper, a hair dye that vanishes in a few hours, and disappearing-graffiti spray paint. There’s a toothpaste that would turn kids’ mouths a bright color until they had brushed for the requisite 30 seconds, and a soap that would do the same for hand washing.
He’s also thinking outside the toy chest, mucking around in the lab on weekends making things like a Swiffer that leaves a momentary trace showing where you’ve Swiffered and a temporary wall paint that would let you spend a few hours with a color before committing to it. The dye’s reach is so great that there are even biotech and industrial uses being discussed.