  In rotating 3He superfluids the Kelvin-Helmholtz (KH) instability of the AB
interface has been found to follow the theoretical model above 0.4 Tc. A
deviation from this dependence has been assumed possible at the lowest
temperatures. Our NMR and thermal bolometer measurements down to 0.2 Tc 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 AB surface tension as a
function of pressure and temperature down to 0.2 Tc.
