  Background: Neutron stars are astronomical systems with nucleons submitted to
extreme conditions. Due to the longer range coulomb repulsion between protons,
the system has structural inhomogeneities. Several interactions tailored to
reproduce nuclear matter plus screened Coulomb term reproduce these
inhomogeneities known as nuclear pasta. These structural inhomogeneities,
located in the crust of neutron stars, can also arise in expanding systems
depending on the thermodynamic conditions (temperature, proton fraction, ...)
and the expansion velocity. Purpose: We aim to find the dynamics of the
fragments formation for expanding systems simulated according to the little big
bang model. This expansion resembles the evolution of neutron stars merger.
Method: We study the dynamics of the nucleons with semiclassical molecular
dynamics models. Starting with an equilibrium configuration, we expand the
system homogeneously until we arrive to an asymptotic configuration (i. e. very
low final densities). We study, with four different cluster recognition
algorithms, the fragment distribution throughout this expansion and the
dynamics of the cluster formation. Results: Studying the topology of the
equilibrium states, before the expansion, we reproduced the known pasta phases
plus a novel phase we called pregnocchi, consisting of proton aggregates
embedded in a neutron sea. We have identified different fragmentation regimes,
depending on the initial temperature and fragment velocity. In particular, for
the already mentioned pregnocchi, a neutron cloud surrounds the clusters during
the early stages of the expansion, resulting in systems that give rise to
configurations compatibles with the emergence of r-proccess. Conclusions: These
calculations pave the way to a comparision between Earth experiments and
neutron star studies.
