The binary collision of two identical liquid droplets is simulated using the moving particle semi-implicit method. We focus on various coalescence and separation mechanisms and the accompanying mass transfer characteristics. A modified surface-tension model is implemented in three-dimensional numerical simulations to study the large deformation processes. Both head-on collision and eccentric impact are investigated, and a mechanism map is established to qualitatively distinguish different regimes of impact. Mass transfer properties are obtained by tracking the movement of particles, which are useful for identifying the mixing rate of the droplets after coalescence or separation as well as the source of the newly formed satellite droplets. A mixing map (in terms of impact speed and impact number) is also established to provide guidelines of pursuing higher efficiency of mixing two liquids using collision. The results qualitatively agree with previous experiments and the versatile numerical protocol may also find applications in studying the free surface flows and interface deformation. © 2009 American Institute of Physics.