  The very early stage of the coalescence of two nuclei is studied and used to
estimate the nuclear viscosity. The time evolution of the neck region has been
simulated by the unified Langevin equation method, which is used in the
analysis of heavy-ion collisions from the approaching stage to the
fusion-fission stage. It is found that the transition from viscous to inertial
coalescence that appeared in the neck growth of macroscopic drops can also be
seen in the present simulation in nucleus-nucleus collisions. The dynamics of
neck growth is analyzed in terms of the hydrodynamical formula and the
viscosity coefficient of nuclear matter is estimated using the analogy of
macroscopic drops.
