There’s been something of an arms race going on at the coffee counter. Nitrogen-infused coffee, cold brew, variations on pour-over coffee and Starbucks' 1.7 million permutations of espresso and milk keep upping the ante almost every year. But the latest trend is layered lattes, in which the normally uniform chocolate-colored beverage is served separated into several distinct and lovely shades of brown.
Making one isn’t too hard. But figuring out why it forms layers is bit more challenging. As Joanna Klein at The New York Times reports, physicists have figured the trick out, and the answer may help in other fields as diverse as manufacturing and oceanography.
As Klein reports, the proper way to make a latte is to dump a shot or two of espresso into the glass before pouring steamed, hot milk into the coffee. But doing it the other way—slowly pouring the coffee into a glass of the steamed milk—often causes layers to form. Retired engineer Bob Fankhauser accidentally made a latte backwards at his home Portland, Oregon, and was intrigued by how layers formed.
He sent an image of his striped concoction to Howard Stone, a fluid dynamics researcher at Princeton, for an explanation. Stone was also flummoxed by the trick, and tasked his graduate student Nan Xue with investigating the physics of the layered latte. They detailed their discovery in the journal Nature Communications. “It's a really intriguing phenomenon,” Fankhauser tells Klein. “There’s no obvious reason that the liquid should organize itself into different density layers.”
According to a press release, Xue started studying the phenomenon by actually making lattes in his lab. Analysis of temperatures and pour rates led Xue to believe that it was possible to describe the physics taking place in the beverage. So the team switched to a less appetizing substitute using heated dyed water seeded with tracer particles to mimic the espresso and warm, relatively dense, saltwater for the milk. Xue then lit up the model with LEDs and hit this setup with a laser, photographing the process of the dyed water mixing with the saltwater.
What the team found is a process called double-diffusive convection—it's the same phenomenon that makes layers in the ocean, Klein reports. In that process, when liquids of differing temperatures and densities, like hot espresso and warm milk, are poured together they don't fully swirl together. Instead, only the boundaries of these layers mix: the hotter liquid heats up a section of the cooler, denser liquid (like milk) causing it to slightly rise, and the cooler denser layer chills out some of the less-dense layer (coffee) causing it to slightly sink. This process creates “convection cells” which flows horizontally, not vertically (which would destroy the layers), resulting in a series of color bands. As Klein reports, the mixture is surprisingly stable, and, at least in coffee, can last hours or even days, as long as the mixture is warmer than the surrounding air.
But it's not just as simple as pouring the coffee through the milk. According to the press release, the rate at which the coffee is poured matters, too. Add it too slowly, and the coffee will evenly mix with the milk, preventing the layers from forming.
While it’s a pretty cool science experiment for bored baristas, it could have implications for manufacturers as well. Stone says in the press release that figuring how to make layers with a single pour could help with processes that currently require building a structure layer by layer.
Detlef Lohse of the University of Twente in the Netherlands, not involved with the study, also says the study may help researchers understand the natural world. “The most awesome finding may be that there is perfect analogy between the layering in a cafe latte, and the known and extremely relevant layering of water with different temperatures and salt concentrations in the ocean," he says in the release.
Which brings to mind another possible coffee variation: the ocean water latte. It has to be better than a Cotton Candy Frappuccino.