  The collision dynamics of two drops of the same liquid moving in the same
direction has been studied numerically. A wide range of radius ratios of
trailing drop and leading drop ($R_r$) and the velocity ratios ($U_r$) have
been deployed to understand the collision outcomes. A volume of fluid (VOF)
based open-source fluid flow solver, Basilisk, has been used with its adaptive
mesh refinement feature to capture the nuances of the interface morphology. The
simulations are analyzed for the evolving time instances. Different collision
outcomes, such as coalescence and reflexive separation with and without the
formation of satellite drops, have been observed for various combinations of
$U_r$ and $R_r$. The study analyzes the evolution of kinetic energy and surface
energy before and after the collision for plausible outcomes. The collision
outcomes are depicted on a regime map with $U_r-R_r$ space, highlighting
distinct regimes formed due to variations in relevant governing parameters.
