Under a black light, highlighter fluid creates ghostly trails as it drips through water. The vortices that form and break into this chandelier-like shape are the result of density differences between the ink and water. Since ink is heavier than water, it sinks, but as the two fluids flow past, they shear one another, forming elaborate shapes. Formally, this is known as the Rayleigh-Taylor instability. While you may be most familiar with it from pouring cream into coffee, it’s also a key to mixing in the ocean and the explosions of supernovas. (Image credit: S. Adams et al.; via Flow Vis)
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This also works with Mrs. Stewart’s Magic Blueing — a blue dye for brightening laundry.
It also works with jello. Make the jello half strength. The sugar makes it denser.
What do you get if you have immiscable liquids that are still close to the same density?