  We investigate in detail the momentum dependence ($MD$) of the effective in
medium Nucleon-Nucleon ($NN$) interaction in the isovector channel. We focus
the discussion on transport properties of the expected neutron-proton ($n/p$)
effective mass splitting at high isospin density. We look at observable effects
from collective flows in Heavy Ion Collisions ($HIC$) of charge asymmetric
nuclei at intermediate energies. Using microscopic kinetic equation simulations
nucleon transverse and elliptic collective flows in $Au+Au$ collisions are
evaluated. In spite of the reduced charge asymmetry of the interacting system
interesting $isospin-MD$ effects are revealed. Good observables, particularly
sensitive to the $n/p$-mass splitting, appear to be the differences between
neutron and proton flows. The importance of more exclusive measurements, with a
selection of different bins of the transverse momenta ($p_t$) of the emitted
particles, is stressed. In more inclusive data a compensation can be expected
from different $p_t$-contributions, due to the microscopic $isospin-MD$
structure of the nuclear mean field in asymmetric matter.
