  In nuclear matter in neutron stars the flavor content (e.g., proton fraction)
is subject to weak interactions, establishing flavor ($\beta$-)equilibrium.
During the merger of two neutron stars there can be deviations from this
equilibrium. By incorporating Urca processes into general-relativistic
hydrodynamics simulations, we study the resulting out-of-equilibrium dynamics
during the collision. We provide the first direct evidence that microphysical
transport effects at late times reach a hydrodynamic regime with a nonzero bulk
viscosity, making neutron star collisions intrinsically viscous. Finally, we
identify signatures of this process in the post-merger gravitational wave
emission.
