  The AT2017gfo kilonova counterpart of the binary neutron star merger event
GW170817 was characterized by an early-time bright peak in optical and UV
bands. Such blue kilonova is commonly interpreted as a signature of weak
$r$-process nucleosynthesis in a fast expanding wind whose origin is currently
debated. Numerical-relativity simulations with microphysical equations of
state, approximate neutrino transport, and turbulent viscosity reveal a new
mechanism that can power the blue kilonova. Spiral density waves in the remnant
generate a characteristic wind of mass ${\sim}10^{-2}~M_{\odot}$ and velocity
${\sim}0.2$c. The ejected material has electron fraction mostly distributed
above $0.25$ being partially reprocessed by hydrodynamic shocks in the
expanding arms. The combination of dynamical ejecta and spiral-wave wind can
account for solar system abundances of $r$-process elements and early-time
observed light curves.
