Last updated January 11, 2018 at 10:25 am
The secret to a perfect latte comes down to the impatience of your barista and some nifty fluid dynamics, according to new research.
Researchers at Princeton University, after having their curiosity piqued by the neat layers of a latte, have investigated how layers develop when espresso is poured into hot milk.
However more than just creating Insta-worthy coffees, the findings will have important implications for industry.
“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,” said Nan Xue who led the research.
When coffee is poured into hot milk, there is an initial mixing of the liquids caused by the turbulence of the pour. However, once allowed to settle, distinct layers of coffee, coffee-milk, and milk emerge.
The reason for the layering is a process called double-diffusive convection. Fluids of different temperatures also create areas of different densities due to thermal expansion (ie, hotter liquids are less dense). Within a mixture, areas of cooler and more dense liquid sink, while lighter, hotter liquids rise.
What you end up with is multiple bands, each band representing a different mix of coffee and milk, giving many different regions of different density and temperature.
What stops these bands from mixing into each other is the way each band behaves compared to those around it. Rather than the sinking and rising of the hot or cold, light or dense coffee, the fluids in the equalised bands begin to flow horizontally within themselves. With the fluid in each band no longer moving up or down into each band on either side, the distinct bands stay in place.
In other words – the layers mix, and then split due to their different temperatures and densities, and then resist mixing by changing the way they interact with each other. The end result is several layers, each with different temperatures (hottest at the top) and densities, depending on how much hot coffee there is in each layer.
The way these layers act means they stay stable for tens of minutes, even hours. They can even resist mixing by stirring.
However, for this effect to occur, the researchers found the liquids needed to be poured at a particular rate. Too slow and the milk will mix too evenly as it flows into the coffee, whereas a faster pour triggers the fluid movements that cause the layer. For once, it might be better to be impatient.
To investigate the mechanics of the process in more detail, the researchers subbed out coffee and milk for something a little less palatable – dyed water in place of the coffee, and salty water for milk.
However the study was not just about coffee. Many industries need to create layered liquids, and working out he ideal way to create bands by flowing liquids into each other could reduce costs and complexity in a 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,” said Howard Stone, who was Xue’s supervisor on the project. “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.”
The knowledge is also useful for ocean studies. The coffee-milk combination is perfectly analogous to the layering of water with different temperatures and salinity in the oceans. Understanding better how those layers form and behave could be useful for climatology and ecology.
Plus, inventive foodies and chefs are sure to get in on it too. No longer will each layer have to be set individually, their relative densities and temperatures can be used to pour them all at once.
So curiosity about coffee has led to techniques that will help industry and understanding the oceans. That’s something to drink to.
The research has been published in Nature Communications