In rotating \(^3\) He superfluids, the Kelvin–Helmholtz (KH) instability of the AB interface has been found to follow the theoretical model above \(0.4 \, T_\textrm{c}\) . A deviation from this dependence has been assumed possible at the lowest temperatures. Our NMR and thermal bolometer measurements down to \(0.2 \, T_\textrm{c}\) show that the critical KH rotation velocity follows the extrapolation from higher temperatures. We interpret this to mean that the KH instability is a bulk phenomenon and is not compromised by interactions with the wall of the rotating container, although weak pinning of the interface to the wall is observed during slow sweeping of the magnetic field. The KH measurement provides the only so far existing determination of the interfacial surface tension at temperatures down to \(0.2 \, T_\textrm{c}\) as a function of pressure.
