The principles and methods of making layered latte

Making a layered latte is not complicated, but do you know why coffee is layered? Joanna Klein from the New York Times interviewed several physicists to try to find the answer.

Klein said that to make a regular latte, you need to pour the whipped milk into the espresso. But if the order is reversed and the coffee is slowly poured into the milk, the coffee will be layered. Former engineer Bob Fankhauser once accidentally achieved coffee layering at home, and he was very interested in the principle behind this phenomenon.

He immediately sent the coffee-making video to his good friend Howard Stone, a fluid dynamics professor at Princeton University. Stone was also quite surprised after watching it, and began to study the physics of layered latte with his student Nan Xue. Later, they also published their research results in the journal Nature Communications. "This phenomenon is really interesting." Fankhauser said, "Liquids should not naturally separate according to density."

During the experiment, in order to find out the truth, they began to make coffee by themselves in the laboratory. By analyzing the temperature and flow rate, they believed that this phenomenon could be explained clearly by the principles of physics. To this end, the research team decided to use dyed hot water with tracer particles to imitate espresso, replace milk with salt water of equivalent density, and light up the liquid with LEDs, and use lasers to record the process of mixing the two liquids.

During the experiment, the team found that this phenomenon is similar to the stratification of seawater, which is called "double diffusion convection". In this phenomenon, when liquids of different temperatures and densities are mixed, such as hot coffee and milk, the two liquids will not merge together immediately. Only a small amount of liquid at the contact surface will merge: the hotter liquid will heat a small amount of colder, denser liquid (i.e. milk), causing it to float slightly; the colder, denser liquid will cool a small amount of hotter, less dense liquid (i.e. coffee), causing it to sink slightly. This process will produce "convection circles", which will only move horizontally, rather than vertically (if vertical movement will destroy the stratification effect), thus producing color bands. Once this phenomenon occurs, it will be very stable and can last for hours or even days, as long as the solution temperature is kept higher than the outside air temperature.

But making a layered latte is more than just pouring coffee into milk. Studies have shown that the speed of pouring the coffee is also very important. If you pour too much, the coffee and milk will blend too much and the layers will not form.