  We present a detailed, 3D hydrodynamics study of the neutrino-driven winds
that emerge from the remnant of a NS merger. Our simulations are performed with
the Newtonian, Eulerian code FISH, augmented by a detailed, spectral neutrino
leakage scheme that accounts for heating due to neutrino absorption in
optically thin conditions. Consistent with the 2D study of Dessart et al.
(2009), we find that a strong baryonic wind is blown out along the original
binary rotation axis within $100$ ms after the merger. We compute a lower limit
on the expelled mass of $3.5 \times 10^{-3} M_{\odot}$, large enough to be
relevant for heavy element nucleosynthesis. The physical properties vary
significantly between different wind regions. For example, due to stronger
neutrino irradiation, the polar regions show substantially larger $Y_e$ than
those at lower latitudes. This has its bearings on the nucleosynthesis: the
polar ejecta produce interesting r-process contributions from $A\sim 80$ to
about 130, while the more neutron-rich, lower-latitude parts produce also
elements up to the third r-process peak near $A\sim 195$. We also calculate the
properties of electromagnetic transients that are powered by the radioactivity
in the wind, in addition to the macronova transient that stems from the dynamic
ejecta. The high-latitude (polar) regions produce UV/optical transients
reaching luminosities up to $10^{41} {\rm erg \, s^{-1}}$, which peak around 1
day in optical and 0.3 days in bolometric luminosity. The lower-latitude
regions, due to their contamination with high-opacity heavy elements, produce
dimmer and more red signals, peaking after $\sim 2$ days in optical and
infrared. Our numerical experiments indicate that it will be difficult to infer
the collapse time-scale of the HMNS to a BH based on the wind electromagnetic
transient, at least for collapse time-scales larger than the wind production
time-scale.
