  The specific viscosity of neutron-rich nuclear matter is studied from the
relaxation time approach using an isospin- and momentum-dependent interaction
and the nucleon-nucleon cross sections taken as those from the experimental
data modified by the in-medium effective masses as used in the IBUU transport
model calculations. The relaxation time of neutrons is larger while that of
protons is smaller in neutron-rich nuclear matter compared with that in
symmetric nuclear matter, and this leads to a larger specific viscosity in
neutron-rich nuclear matter. In addition, the specific viscosity decreases with
increasing temperature because of more frequent collisions and weaker Pauli
blocking effect at higher temperatures. At lower temperatures the specific
viscosity increases with increasing density due to the Pauli blocking effect,
while at higher temperatures it slightly decreases with increasing density as a
result of smaller in-medium effective masses at higher densities.
