Coalescence of viscous drops with surfactants

Drop coalescence is a familiar event in both nature and technology, from spray and atomization systems to the production of food emulsions. It is well known that surfactants – which are ubiquitous contaminants and routinely used as industrial additives – play an important role in preventing drop coalescence. Little is known, however, about the mechanisms of coalescence after surfactant-laden drops make contact. Here we analyze these mechanisms using direct numerical simulation. Results show that due to the uneven contraction of the interface the surfactant accumulates on the liquid meniscus bridge joining the drops. Simulations further reveal that the accumulation is hampered by Marangoni stresses induced, in turn, by the uneven surfactant distribution. These entwined mechanisms control the rate at which the drops coalesce by modulating the pull of surface tension on the meniscus bridge.

► We model the coalescence of viscous drops with surfactants.
► Surfactant accumulates on the liquid meniscus bridge between the drops.
► The accumulation of surfactant is hampered by Marangoni stresses.
► The balance between these coupled mechanisms controls the coalescence rate.