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Mixing a coffee drink is a surprisingly complicated science

Inspiration can come from the most random of sources. For Howard Stone, a professor of mechanical and aerospace engineering at Princeton University, the inspiration for his new research project on the layered structure of coffee came from an emailed picture of a layered coffee drink he received unsolicited. Small details can also have big implications, and Stone’s research that followed may help explain the heat- and salinity-dependent flows of Earth’s oceans.

Nan Xue, a graduate student and lead author of the paper, began investigating these coffee layers by preparing lattes in the lab using store-bought coffee and milk. After much trial and error, he found that he was only able to achieve the characteristic café latte layering by staying within certain parameters, such as temperatures and pour rates. This showed that there must be underlying, quantifiable physics that are involved in this liquid structure formation.

“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,” says Nan Xue.

Using a panel of light-emitting diodes and a camera – as well as a coffee and milk substitute (dyed water for coffee and salty, denser water for milk) – the researchers captured the movement of fluids within the mixture. They also seeded the concoction with tracer particles, which scattered light from a green laser beam, allowing them to further track the drink’s internal dynamics.

The results showed that that primary mechanism behind layering is a phenomenon called double-diffusive convection. This occurs when stacked-up fluids of different densities are made to mix their contents, and exchange heat through the movement of their constituent materials. Within a given mixture, lighter, hotter liquids rise, while cooler, denser liquids sink. When the local density in a region within the mixture approaches equilibrium, the sinking and rising stops. Thus, the fluid in that region flows horizontally, rather than vertically. This creates distinct bands, or layers.

Perfecting techniques for creating sought-after layers of liquids flowing into each other has implications that stretch beyond simply pouring coffee. Improving our understanding of the mechanics of this process could reduce costs and complexity in a wide range of applications.

“From a manufacturing perspective, a single pouring process is much simpler than the traditional sequential stacking of layers in a stratified product,” explains Stone. “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.”

Detlef Lohse, a professor of fluid mechanics at the University of Twente in the Netherlands who was not involved in this study, believes that the results of this research are quite significant. “I think it will have bearing on various industrial flows and mixing procedures in so-called process technology, in which mixing of fluids with different densities by the injection of one into the other is omnipresent,” he explains.

Furthermore, Lohse points out that this study may help better explain the heat- and salinity-dependent flows of water in Earth’s oceans. This may have major implications in climatology and ecology. “The most awesome finding may be that there is perfect analogy between the layering in a cafe latte,” says Lohse, “and the known and extremely relevant layering of water with different temperatures and salt concentrations in the ocean.”

Who knew? The physics and fluid mechanics of your morning cup of coffee could be deeply connected to the phenomena occurring in our vast oceans.

By Connor Ertz, Staff Writer

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