Marangoni vs Weber (and Reynolds): Complex multiphase flows in the presence of surfactants

Surface active agents (surfactants) are present either as chemical additives or as contaminants in flows involving interfaces. These molecules adsorb at these interfaces and lower the interfacial tension. Interfacial gradients of their concentration lead to interfacial tension gradients, which, in turn, induce Marangoni stresses. The presence of these Marangoni stresses has a profound effect on the interfacial dynamics, which we illustrate numerically through several exemplar flows that include turbulent jets, falling films, bursting bubbles, and retracting filaments. These flows are characterised by Reynolds, Weber, and Marangoni parameters that reflect the delicate interplay amongst inertial, viscous, and capillary forces, as well as Marangoni stresses. The physico-chemical effects related to surfactant elasticity, diffusivity, solubility, and sorption kinetics are also taken into account. We use an interface-tracking/level-set approach to capture accurately the interfacial dynamics which are punctuated by topological transitions in certain cases. We highlight the mechanisms underlying the surfactant-induced effects, which are often related to interfacial rigification and associated qualitative changes to the emergent flow structures. Open problems which constitute departure points for future research are also highlighted.