  Understanding the interface dynamics in non-equilibrium quantum systems
remains a challenge. We study the interface dynamics of strongly coupled
immiscible binary superfluids by using holographic duality. The full nonlinear
evolution of the binary superfluids with a relative velocity shows rich
nonlinear patterns toward quantum turbulence, which is reminiscent of the
quantum Kelvin-Helmholtz instability. The wave number of the fast growing modes
$k_0$ extracted from the interface pattern yields a non-monotonic dependence of
the relative velocity, independent of the temperature and interaction. The
value of $k_0$ first increases with the velocity difference and then decreases,
which stands in sharp contrast to the results of mean-field theory described by
the Gross-Pitaevskii equation and is confirmed by using the linear analyses on
top of the stationary configuration. We uncover that the critical velocity
associated with the maximum correspond to the case when the mean separation of
vortices generated by interface instabilities becomes comparable to the vortex
size, which could be a universal physical mechanism at strongly interacting
superfluids and is directly testable in laboratory experiments.
