If you pour coffee into warm milk at just the right speed, the laws of physics will make the two liquids arrange themselves into an elegant series of layers shading from white to dark brown, according to a new study that may have applications far beyond the barista’s realm.
The secret, Howard Stone of Princeton University, US, and colleagues write in Nature Communications, is lies in a phenomenon known as “double-diffusion convection”, which occurs when variations in density and temperature work together to create self-sustaining loops of flowing liquid.
The researchers started by pouring coffee into a glass of milk, with both at a temperature of 50 degrees Celsius. The coffee is slightly less dense than the milk, so it has a tendency to float on top.
If it is poured quickly enough, however, it will penetrate more deeply into the milk and the two liquids will form a turbulent and chaotic mixture.
Now, as the mixture begins to cool down, double-diffusion convection comes into play. The liquid at the top of the glass contains more coffee so it is less dense than the liquid below it, creating a vertical density gradient. And the liquid at the sides of the glass, which can radiate heat to the air outside, is cooler than the liquid in the centre of the glass.
The cooling liquid at the sides becomes denser and starts to fall down until it meets liquid that has slightly higher density due to the proportions of its milk-coffee blend. At that point, the falling flow of liquid turns inward and runs horizontally to the centre of the glass. There it is pushed upward, until it runs into the horizontal flow of liquid at the bottom of the layer above and flows out to the side of the glass again.
This process divides the liquid up into a series of layers containing different concentrations of coffee. If the glass is not disturbed, the layers can stay in place for 20 minutes or more.
Stone and his team replicated the results using a mix of salt water and fresh water, which has a similar ratio of densities to milk and coffee, and also conducted computer simulations that confirmed the mechanics of the process.