On Kelvin-Helmholtz instability in superfluids
  The Kelvin-Helmholtz instability in superfluids is discussed, based on the first experimental observation of such instability at the interface between superfluid 3He-A and superfluid 3He-B (cond-mat/0111343). We discuss why (i) the Kelvin-Helmholtz criterion, (ii) the Landau critical velocity for nucleation of ripplons, and (iii) the free energy consideration, all give different values for the instability treshold. 
G.E. Volovik