My Cup Runneth Down

The Rayleigh-Taylor Instability is responsible for fingers of color formed as water tinted with food coloring flows down through ordinary tap water, which is slightly less dense that the dyed water. Pictured experiment provided by James Riordon, AIP

It might seem intuitively obvious that a layer of dense liquid resting on a less dense liquid is an unstable situation. But what isn't so obvious is the complex way that liquids arranged in this manner tend to move. The Rayleigh-Taylor Instability is a phenomenon that leads to beautiful, and mathematically intricate, structures as a more dense liquid flows down through a less dense liquid.

The Rayleigh-Taylor Instability has been invoked to explain how water spills from a glass, the dynamics of magma percolating beneath the Earth's crust, and features in space such as the Eagle Nebula – which was recently captured in striking images collected by the Hubble Space Telescope. The instability is also a vexing problem for fusion physicists, who often find that it limits their ability to compress or confine fusion plasmas.

Countless computer hours have been dedicated to simulating the Rayleigh-Taylor Instability in universities and labs around the world. Due to the difficulty of placing a layer of dense fluid on top of another fluid, experimental simulations generally require shock tubes, high-energy lasers, or other specialized equipment. But, a simple and inexpensive demonstration of the Rayleigh-Taylor Instability can be carried out in a wine glass.

A few drops of food coloring added to tap water forms a colorful liquid that is just slightly more dense than ordinary water. If the colored water is heated to boiling in a microwave, its density can be reduced to the point that it will float in a layer at the top of a glass of ordinary water that has been chilled nearly to its freezing point. As the colored water cools and the clear water heats up, the density of the dyed water eventually exceeds that of the water below it, and the Rayleigh-Taylor Instability leads to gaudy fingers of color that reach down to the bottom of the glass.

Link: Gaseous Pillars: M16