  Simple observations of evaporating solutions reveal a complex hierarchy of
spatio-temporal instabilities. We analyze one such instability suggested by the
qualitative observations of Du and Stone and find that it is driven by a novel
variant of the classical {\em morphological instability} in alloy
solidification. In the latter case a moving solid-liquid interface is
accompanied by a solutally enriched boundary layer that is thermodynamically
metastable due to {\em constitutional supercooling}. Here, we consider the
evaporation of an impure film adjacent to a solid composed of the nonvolatile
species. In this case, constitutional supercooling within the film is created
by evaporation at the solution-vapor interface and this drives the corrugation
of the solid--solution interface across the thickness of the film. The
principal points of this simple theoretical study are to suggest an instability
mechanism that is likely operative across a broad range of technological and
natural systems and to focus future quantitative experimental searches.
