  We study the interface dynamics in immiscible binary superfluids using its
holographic description, which naturally consists of an inviscid superfluid
component and a viscous normal fluid component. We give the first theoretical
realization of interface instability for two superfluid components moving with
identical velocity, providing a quantum analog to the flapping of flags that is
common in daily life. This behavior is in sharp contrast to the one from
Gross-Pitaevskii equation for which no such co-flow instability develops in an
isolated uniform system because of Galilean invariance. The real time evolution
triggered by the dynamical instability exhibits intricate nonlinear patterns
leading to quantum turbulence reminiscent of the quantum Kelvin-Helmholtz
instability. Moreover, we show that such interface dynamics is essentially
different from the Landau instability for which the frictionless flow becomes
thermodynamically unstable above a critical superfluid velocity. Our study
uncovers the rich interface dynamics of quantum fluids and the emergence of
complex flow phenomena.
