Heavy-ion collisions at the Fermi energies(10100 AMeV)attract much attention on several topical issues in nuclear physics,i:e:,spinodal multifragmentation,liquid-gas phase transition,properties of highly excited nuclei,symmetry energy in the domain of subnormal densities,etc.The isospin dynamics in the Fermi-energy heavy-ion collisions is related to several interesting topics,e:g:,in-medium nucleon-nucleon cross sections,cluster formation,isotopic distribution,the symmetry energy in dilute nuclear matter,etc.Much progress has been made in constraining the density dependence of symmetry energy in the domain of sub-saturation densities both in theories and in experiments.Different mechanisms can coexist and are correlated in heavy-ion collisions at the Fermi energies,such as projectile or target fragmentation,neck emission,preequilibrium emission of light clusters(complex particles),fission of heavy fragments,multifragmentation,etc,in which the isospin dependent nucleon-nucleon potential dominates the dynamical processes.The time scales from the dynamical and pre-equilibrium emissions to the statistical decay of excited systems at equilibrium and the isospin effect in neck fragmentation were investigated experimentally[1].Transport models are needed to understand the mechanisms correlated with the emission time and the dependence on incident energy,collision centrality and reaction systems.
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