  An analytical expression of the coefficient of restitution for viscoelastic
materials is derived for the viscous-dominant case, such as collisions of
polymeric melt. The recently proposed normal impact force model between two
colliding viscoelastic droplets is employed. To analytically solve the
nonlinear differential equation of deformation caused by the other droplet, a
perturbation method is applied. By combining the forward collision and the
inverse collision, we can get an analytical expression for the coefficient of
restitution, which can also be used to further analyze other properties related
with the viscoelasticity.
