Ripples and Ripples
  We study the morphological evolution of surfaces during ion sputtering and we compare their dynamical roughening with aeolian ripple formation in sandy deserts. We show that, although the two phenomena are physically different, they must obey to similar geometrical constraints and therefore can be described within the same theoretical framework. The present theory distinguish between atoms that stay bounded in the bulk and others that are mobile on the surface. We describe the excavation mechanisms, the adsorption and the surface mobility by means of a continuous equation derived from the study of dune formation on sand. This approach can explain the different dynamical behaviors experimentally observed in metals or in semiconductors and amorphous systems. We also show that this novel approach can describe the occurrence of ripple rotation in the $(x,y)$ plane induced by changes in the sputtering incidence angle. 
T. Aste and U. Valbusa