Latte Art

Physics
Latte art is a mixture of two colloids: the crema, which is an emulsion (liquid-in-liquid) of coffee oil and brewed coffee (water); and the microfoam, which is a foam (air-in-liquid) of air in milk. (Milk itself is an emulsion of butterfat in water, while coffee is a solution of coffee solids in water.) Neither of these colloids are stable - crema dissipates from espresso, while microfoam separates into drier foam and (liquid) milk - both degrading significantly on the order of minutes, and thus latte art lasts only briefly.