Foam patterns on espresso

It looks to me as a capillary-gravity wave instability, caused by the relative motion of air with respect to the surface. Ripples appear on smooth fluids when a wind-like current blows over them, but will die quickly if it stops. The restoring force that allows them to propagate is surface tension.

Capillary waves are common in nature, and are often referred to as ripples. The wavelength of capillary waves in water is typically less than a few centimeters.

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.