Latte showing banded layers.

Technique, not espresso quality, conjures up the stripes in a glass of latte. Credit: Pixabay (CC0)

Physics

How warm milk generates latte layers

High-speed espresso injection creates that banded look.

It takes a quick pour to create the stripes in a layered latte.

When a barista makes a latte, distinct horizontal layers sometimes form as the espresso settles into the denser milk. To investigate this process, Howard Stone of Princeton University in New Jersey and his colleagues ran computer simulations and experiments that involved injecting warm dyed water into denser salt water. They found that if espresso is injected fast enough into a glass of warm milk, the espresso–milk mixture closest to the wall of the glass cools, becomes denser and sinks until it reaches a layer of the same density. At that point, it stops sinking and begins to circulate horizontally, forming multiple convection cells that can retain their structure for hours.

The team demonstrated a way to use this technique to create layered gels that could be useful for cell cultures. The method could also be applied in manufacturing and tissue engineering, the authors write.