  On the proper timescale, amorphous solids can flow. Solid flow can be
observed macroscopically in glaciers or lead pipes, but it can also be
artificially enhanced by creating defects. Ion Beam Sputtering (IBS) is a
technique in which ions with energies in the 0.1-10 keV range impact against a
solid target inducing defect creation and dynamics, and eroding its surface
leading to formation of ordered nanostructures. Despite its technological
interest, a basic understanding of nanopattern formation processes occurring
under IBS of amorphizable targets has not been clearly established, recent
experiments on Si having largely questioned knowledge accumulated during the
last two decades. A number of interfacial equations have been proposed in the
past to describe these phenomena, typically by adding together different
contributions coming from surface diffusion, ion sputtering or mass
redistribution, etc. in a non-systematic way. Here, we exploit the general idea
of solids flowing due to ion impacts in order to establish a general framework
into which different mechanisms (such as viscous flow, stress, diffusion, or
sputtering) can be incorporated, under generic physical conservation laws. As
opposed to formulating phenomenological interfacial equations, this approach
allows to assess systematically the relevance and interplay of different
physical mechanisms influencing surface pattern formation by IBS.
