  We study out-of-thermodynamic equilibrium effects in neutron star mergers
with 3D general-relativistic neutrino-radiation large-eddy simulations. During
merger, the cores of the neutron stars remain cold ($T \sim$ a few MeV) and out
of thermodynamic equilibrium with trapped neutrinos originating from the hot
collisional interface between the stars. However, within ${\sim}2{-}3$
milliseconds matter and neutrinos reach equilibrium everywhere in the remnant.
Our results show that dissipative effects, such as bulk viscosity, if present,
are only active for a short window of time after the merger.
