Direct numerical simulation of three-dimensional liquid jet breakup with surfactant*

Surfactant-laden jet flows are studied using a massively-parallelised, direct numerical simulation code, which uses a hybrid technique based on the front-tracking and the level-set methods. We focus on isolating the effect of surfactant-induced Marangoni stresses on the complex interfacial dynamics. The interface-tracking nature of the numerical method employed facilitates the faithful modelling and simulation of the spatio-temporal evolution of the surfactant concentration field for both soluble and insoluble surfactant species. This type of jet flow features (i) the formation of a ‘mushroom’-like shape exhibiting ‘roll-up’ that are driven by the density contrasts between the two phases; (ii) Kelvin-Helmholtz instability due to the velocity contrast between the phases; (iii)  atomization inside and outside the liquid jet preceded by the formation of holes, ligaments, and droplets pinch-off events. The addition of surfactant slows down the breakup process, which is due to the presence of surfactant-induced Marangoni stresses. A full parametric study is presented, and the relevant mechanisms underlying the flow phenomena are elucidated.