  With most of the focus to date having been on the coalescence of freely
suspended droplets, much less is known about the coalescence of sessile
droplets, especially in the case of droplets laden with surfactant. Here, we
employ large-scale molecular dynamics simulations to investigate this
phenomenon on substrates with different wettability. In particular, we unravel
the mass transport mechanism of surfactant during coalescence, thus explaining
the key mechanisms present in the process. Close similarities are found between
the coalescence of sessile droplets with equilibrium contact angles above
90{\deg} and that of freely suspended droplets, being practically the same when
the contact angle of the sessile droplets is above 140{\deg}. Here, the initial
contact point is an area that creates an initial contact film of surfactant
that proceeds to break into engulfed aggregates. A major change in the physics
appears below the 90{\deg} contact angle, when the initial contact point
becomes small and line-like, strongly affecting many aspects of the process and
allowing water to take part in the coalescence from the beginning. We find
growth exponents consistent with a 2/3 power law on strongly wettable
substrates but no evidence of linear growth. Overall bridge growth speed
increases with wettability for all surfactant concentrations, but the speeding
up effect becomes weaker as surfactant concentration grows, along with a
general slowdown of the coalescence compared to pure water. Concurrently, the
duration of the initial thermally limited regime increases strongly by almost
an order of magnitude for strongly wettable substrates.
