The instability of the surface of helium crystal in a superfluid flow

The problem on crystal growth under conditions of normal incidence of fluid on the boundary is investigated for stability. The threshold velocity of the emergence of instability is found; at low temperatures, this velocity proves to be much lower than the sound velocity. The stability is examined of the shape of cylindrical crystal in a fluid flow parallel to the crystal axis. The behavior of the atomically rough surface of crystal helium is experimentally investigated in a jet of fluid in the temperature range from 1 to 1.4 K, where the emergence is observed of an instability of the type previously predicted by Kagan, as well as by Nozieres and Uwaha. Observations reveal that, below the roughening transition, the (0001) basal face is stable in a jet of fluid.