6.3 Differentially-rotating flows in superfluid helium

At the end of Section 4.2.2 we briefly described an analogue model based on the ripplons in the surface separating two differentially-moving superfluids, in particular an AB-interphase in 3He. These interphases are being produced in Helsinki’s Low Temperature Lab [76Jump To The Next Citation Point, 77, 201, 202]. The AB-interphase is prepared in a small quartz cylinder (3 mm radius times 11 cm long) inside a rotating cryostat. The 3He-A is rotating with the cryostat while the 3He-B remains at rest with respect to the lab. Among other things, in this setting the critical values at which instabilities appear as functions of the temperature, and the nature of these instabilities, are being investigated. These instabilities are related to the appearance of an ergoregion in the analogue metric for ripplons and to the Kelvin–Helmholtz instability [657]. In particular, this has been the first time that the Kelvin–Helmholtz instability has been observed in superfluids [76]. The nature of the instability in these experiments is controlled by the difference in velocities between the normal and superfluid components of the flow. It still remains to further lower the ambient temperature so as to probe the nature of the instabilities in the absence of any normal fluid component.
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