A concept of isospin blocking in the process of isospin diffusion in heavy-ion collisions is raised. Generally, in the process of isospin diffusion, isospin asymmetry would diffuse from the place with large asymmetry to the place with small asymmetry. However, our study shows that the isospin diffusion could be blocked in cases where the local value of the symmetry energy is larger. We dub this phenomenon “isospin blocking”. To check this behavior, in the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model, isospin diffusions in the isotope $\mathrm{Sn}+\mathrm{Sn}$ reactions at 270 MeV/nucleon are studied. It is shown that the value of the after-diffusion asymmetry is distinctly blocked if the local symmetry energy is large. The effects of the isospin blocking on the isospin asymmetry of dilute and dense matter and the final ${\pi }^{-}/{\pi }^{+}$ ratio in heavy ion collisions are demonstrated.

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• Low & intermediate energy heavy-ion reactions
• Nuclear matter