Foams are very complex and interesting fluids," Emilie Dressaire, an engineer who leads the Particles, Interface and Fluids Lab at New York University, told Live Science in an email. This makes it difficult to know exactly what's going with latte art without studying it specifically. [10 Things You Need to Know About Coffee]
One thing is for certain: Milk doesn't turn into foam by itself.
"Simply put, we steam the milk for two reasons," Lorenzo Perkins, owner of Fleet Coffee in Austin, Texas, and a former chairman of the Barista Guild of America, told Live Science.
Properly steamed milk tastes sweeter, and the texture is thick and luxurious. "Making it sweet and creamy is really the goal," Perkins said.