Multiphase flows are of central importance to a wide range of industrial applications and environmental settings. Examples of these include mixing in stirred vessels and static mixers, flows in micro-channels and microfluidics devices, falling films for CO2 capture, and aerosol formation via bubble bursting through interfaces in the oceans. Some of these flows feature the presence of surface-active agents (surfactants), present either by design or as contaminants. Furthermore, multiphase flows are often punctuated by topological transitions related to the coalescence of dispersed drops or bubbles, and the breakup of threads or ligaments. Here, we provide a few examples of interest to the JFM community but focus on drop impact on hydrophobic substrates in the presence of surfactants above the critical micelle concentration. Our model accounts for the spatio-temporal evolution of the surfactants along the interface and within the bulk; the bulk and interfacial species are fully-coupled via sorptive fluxes. Micellar formation and breakup are also accounted for, and the surfactant dynamics are coupled to the flow through the dependence of the surface tension on the local interfacial surfactant concentration. Our numerical procedure is based on the use of a hybrid interface-tracking/level-set approach. The results of our parametric study help identify the various physical mechanisms underlying the observed flow phenomena.
