In this paper, the two dimensional coalescence of two stationary head-on droplets with equal diameters are numerically investigated using the lattice Boltzmann method (LBM). Droplets are considered to have a circular shape with a specified gap size between their free surfaces. Coalescence is initiated when two droplets come into contact and form a liquid bridge, which then starts to grow due to surface tension. It is shown that for the coalescence to occur, the gap size has to be less than a certain limit called critical gap size. Finally, the effects of important parameters including surface tension, density and viscosity ratio of two phases on the critical gap size are studied. The results show that increasing the surface tension value and vapor to liquid density ratio increases the critical gap size. In contrast, increasing the kinematic viscosity of surrounding vapor decreases the critical gap size.

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