Study details the physics of a café latte

New research by scientists sat Princeton University has revealed the physics underlying the delicious layers of a café latte.

Scientists have discovered the physics that account for the layering of coffee and milk in a cafe latte. Photo by Pixabay/CC
Scientists have discovered the physics that account for the layering of coffee and milk in a cafe latte. Photo by Pixabay/CC

“The structure formation in a latte is surprising because it evolves from the chaotic, initial pouring and mixing of fluids into a very organized, distinct arrangement of layers,” researcher Nan Xue said in a news release.

Xue is a grad student student working in the lab of Howard Stone, professor of mechanical and aerospace engineering at Princeton.

Though inspired by an everyday drink order, Xue’s investigation yielded insights into the layering of fluids — insights that could have a variety of useful applications.

“From a manufacturing perspective, a single pouring process is much simpler than the traditional sequential stacking of layers in a stratified product,” Stone said. “In one application of this study, we are exploring the physics behind making a whole layered structure with one step, rather than one-by-one stacking of the layers.”

Xue initially experimented with store-bought coffee and milk but struggled to maintain consistency as he attempted to recreate a latte’s layers. To simplify the experimentation process, he used dyed water as stand-in for hot coffee and salty, denser water as a substitute for warm milk.

Xue and his research partners mixed racer particles into the two liquids and used light-emitting diodes and a camera to track the movement of fluids during the mixing and layering process. The collected data — which scientists compared with models of intermixing liquids — revealed the physical principles underlying the phenomenon.

As revealed by their analysis, the most important mechanism is double-diffusive convection. The mechanism describes the flow patterns as different density liquids mix and heat is diffused. At first vertical flows occur, but as temperature and density differences begin to reach equilibrium, flows begin to move horizontally and layers develop.

The mechanism is dependent on the temperature and density difference of the liquids and — in the case of a café latte — the speed at which the milk is poured into the coffee. If poured too slowly, the milk and coffee mix too evenly and distinct layers fail to form.

Researchers are now working on understanding how double-diffusive convection might work in other liquids and semi-solids.

That Xue was able to replicate the layering process of a café latte using water with different levels of salt suggests the findings — published this week in the journal Nature Communications — could help scientists better understand currents and upwelling in the ocean.

By Brooks Hays