Four　collision　behaviors　are　observed　for　droplet　pairs　at　a　T-junction　and　phase　diagrams　are　constructed　by　considering　the　arriving　distance　difference,　the　droplet　length　and　the　capillary　number.　Critical　conditions　for　both　coalescence　and　splitting　are　shown.　The　main　coalescence　regime　is　varied　for　asynchronous　droplet　pairs　and　the　cause　of　the　coalescence　which　occurs　while　two　droplets　are　pulled　apart　is　confirmed.　Movements　of　the　front　caps　of　various　droplet　pairs　are　exhibited　to　show　their　different　degrees　of　competitions　to　penetrate　into　the　downstream　channel　and　the　advantage　that　favors　coalescence　is　acquired　by　comparison.　(C)　2018　The　Korean　Society　of　Industrial　and　Engineering　Chemistry.　Published　by　Elsevier　B.V.　All　rights　reserved.