Tia Maria and the physics of getting liquored up

Pouring a thin layer of cream on top of Tia Maria coffee liqueur results in a in a constantly-roiling cell pattern on the surface of the cream. What's driving the motion?

If you're looking to get introspective, or hypnotized, while getting hammered, here's a drink that will keep your drunk eyes and your drunk mind engaged. Pour a little liqueur into a glass. When it settles, pour some cream on top of it – pouring over the back of a spoon, so it doesn't splash down into the alcohol is best.

Within a few minutes, the cream will change from a uniform white layer into many darker cells. If you keep watching, you'll see that the cells seem to split and roll into bigger and more complicated patterns.

This is the result of convection and surface tension. The cream that's exposed to the alcohol picks up a little alcohol itself, and it does it in a special way. Instead of just mixing with the cream, each little packet of cream sucks in some alcohol. This expands the cream packet and makes it less dense than the cream at the surface. Since less dense stuff rises up, this alcohol-cream packet shoots upwards.

At the surface of the cream, the molecules stick together via surface tension, forming a kind of membrane. This membrane is stretched as the alcohol pushes upwards, much the way a balloon would be stretched out if it were too inflated. The layer of cream on top of the pushing alcohol-cream gets thinner and thinner. When a balloon bursts, it generally doesn't shatter into pieces. Instead, the part that was overstretched gives, and snaps back to the rubber that surrounded it. That's what happens to the cream. It gets pulled towards the sides.

This forms the first grouping of cells, but it doesn't explain why they keep boiling. Alcohol evaporates. Fat doesn't. When the little ball of cream, loaded with alcohol, gets to the surface, the alcohol evaporates out into the air. The cream packet gets denser again and sinks back down to where it can pick up alcohol again. This creates a constant motion, and a constant mixing of the two liquids.

Via The Times Online and Alice in Galaxyland and New Scientist.