Here, the coalescence of similar and dissimilar droplets has been investigated in microfluidics while varying the rheological and physical properties (e.g. viscosity, density) of the fluids. Our data indicate that in early regimes, the dynamics of coalescence depends on the properties of the inner fluid, which can generate a dominant viscous or inertial regime, while the outer fluid controls the activation or inhibition of the merging droplets. Besides, the models describing the mechanism in vacuum or in a very low viscous outer fluid become invalid when one of those conditions fails. An accurate modification resulted from accounting for the viscosity ratio of the inner and outer fluid. A phase diagram showing the crossovers into the different possible late-time dynamics identifies that, depending on the ratio of the viscosities of the inner and outer fluid, the coalescence develops by an inertial limited viscous, pure viscous or inertial regime. (C) 2019 Elsevier Ltd. All rights reserved.