Waves on the He-II Surface, Excited by a Heat Flux in the Bulk

Abstract

The conditions for the occurrence of an instability generated on the free surface of superfluid He-II by a constant or variable heat flux Q in the He-II bulk have been experimentally studied. This instability is a quantum analog of the Kelvin–Helmholtz instability; it is accompanied by the occurrence of low-frequency oscillations on the surface and develops when the maximum velocity of the counterflow of the normal and superfluid components under the surface reaches the threshold value. It is shown that the large variance in the estimates (differing by a factor of 50–100) of the threshold heat flux Qthr (waves occur on the free He-II surface when the threshold is exceeded) in three series of experiments can be explained by different cross-sectional areas and numbers of channels in the lateral walls, through which the normal He-II component transfers heat from the immobile cell into the external helium bath (i.e., the significant difference in the boundary conditions). It is established that, switching on a heater, one can implement conditions in the liquid bulk that are sufficient for observing simultaneously the parametric Faraday instability and Kelvin–Helmholtz instability on the free He-II surface in a hermetically closed oscillating cell cooled from outside by superfluid He-II at a constant temperature.