  Within nuclear thermodynamics and an isospin-dependent transport model we
investigate respective roles of the nuclear mean field and the 2-body
stochastic scattering on the evolution of density and isospin fluctuations in
either mechanically or chemically unstable regions of neutron-rich matter. It
is found that the mean field dominates overwhelmingly the fast growth of both
fluctuations, while the 2-body scattering influences significantly the later
growth of the isospin fluctuation only. Moreover, both fluctuations grow in
mechanically unstable systems, while only the density fluctuation grows
significantly in chemically unstable ones. Furthermore, the magnitude of both
fluctuations decreases with the increasing isospin asymmetry because of the
larger reduction of the attractive isoscalar mean field by the stronger
repuslive neutron symmetry potential in the more neutron-rich matter. Finally,
several experimental measurements are proposed to test these findings.
