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 $^3$He-A and superfluid $^3$He-B (R. Blaauwgeers, V. B. Eltsov, G. Eska et al., cond-mat/0111343). We discuss why the Kelvin—Helmholtz criterion, the Landau critical velocity for nucleation of ripplons, and the free energy consideration all give different values for the instability treshold.