# Foam patterns on espresso

When just prepared, an espresso has a thin layer of foam, known as crema, on its surface. As I move my drink from the bar to a table, the motion destroys the homogeneity of the layer, but I've noticed that it's common to get regularly-spaced stripes of foam on the surface. The stripes of foam have wavelength of about $1\mathrm{mm}$.

Is there an explanation for these patterns and for this particular wavelength, or are they just formed by chance? (I don't have enough money to buy lots of espressos to do experiments...)

Edit: Here's a photo. The crema was initially uniform, but formed the stripes after I gently moved the cup back and forth and blew on the surface.

The wavelength of such waves depends on the surface tension of the fluid, besides of course $$g$$. The derivation of the general dispersion relation is in general non-trivial, particularly in a bounded domain, where additional lengt scales and curvature appear.