  Droplet coalescence is an important process in nature and various
technologies (e.g. inkjet printing). Here, we unveil the surfactant
mass-transport mechanism and report on several major differences in the
coalescence of surfactant-laden droplets as compared to pure water droplets by
means of molecular dynamics simulation of a coarse-grained model. Large scale
changes to bridge growth dynamics are identified, such as the lack of multiple
thermally excited precursors, attenuated collective excitations after contact,
slowing down in the inertial regime due to aggregate-induced rigidity and
reduced water flow, and a slowing down in the coalescence rate (deceleration)
when surfactant concentration increases, while at the same time we also confirm
the existence of an initial thermal, and a power-law, inertial, regime of the
bridge growth dynamics in both the pure and the surfactant-laden droplets.
Thus, we unveil the key mechanisms in one of the fundamental topological
processes of liquid droplets containing surfactant, which is crucial in
relevant technologies.
