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How layers in a latte form

Joanna Klein, The New York Times Dec 28 2017, 11:09 IST
Handout photos showing espresso poured into heated milk, and how layers of the two liquids form and separate over time. Researchers at Princeton University observed a phenomenon called double-diffusive convection, which creates different density layers in the liquids. Nan Xue, Sepideh Khodaparast and Howard A. Stone via The New York Times

Handout photos showing espresso poured into heated milk, and how layers of the two liquids form and separate over time. Researchers at Princeton University observed a phenomenon called double-diffusive convection, which creates different density layers in the liquids. Nan Xue, Sepideh Khodaparast and Howard A. Stone via The New York Times

Any good barista will tell you that if you want to make a nice latte you pour milk into the espresso - not the other way around. But there's another style of latte out there, too - the layered latte, or #layeredlatte as you'll find on Instagram. Created by accident, or by baristas experimenting with new drinks, these striped beverages start with a glass of heated milk and then pour in the espresso.

Bob Fankhauser, a retired engineer in Portland, Oregon, accidentally created his own layered latte at home and wanted to know why these pretty layers form. Last year, Bob sent an email including photos of his accidental layered lattes to Howard Stone, a chemical engineer who studies fluid dynamics at Princeton University, USA and inspired him and a graduate student to test this out. The team published their results in Nature Communications on December 12. After recreating the latte with their own espresso and milk, the team created a simulated coffee drink, injecting heated, dyed freshwater into heated, denser saltwater to test the scientific parameters that make this spontaneous layering possible. Pouring hot espresso into warm milk at a certain speed, they found, induced an interaction between temperature and density that caused the drink to separate into layers of different densities.

The same basic phenomenon, called double-diffusive convection, creates layers of water in the ocean. Nan Xue, a graduate student in Howard's lab who led the study, found that even if you disturb the layers with a gentle stir, they will reform and stay put - for minutes, hours, even days. As long as the mixture is still warmer than the air around it, the stirring creates another density gradient, similar to that produced by pouring. But stir after the latte reaches room temperature - bye-bye stripes. To create your own layered latte, pour hot espresso over a spoon into a tall glass of milk of about the same temperature. Wait a few minutes for the layers to form as the liquid cools.

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