Thin film flows are at the nexus of a large number of industrial applications that include manufacturing of fast-moving consumer goods, enhanced oil recovery, electronics, coating processes, and many more. The multiphase flow dynamics of such systems are affected by a myriad of physics (e.g., capillarity, complex rheology, heat and mass transfer, phase change, gravity, intermolecular and electro-magnetic interactions, and others). The physical complexity of thin film flow has excited the scientific community for decades. Over the years, many experimental efforts in the field have unravelled a multitude of challenges associated with the multi-scale nature of the physical phenomenon. Most attempts fall short of scrutinising these problems fully. Additionally, the complex topological structures of these flows are often influenced by naturally occurring, or deliberately placed surface-active species, resulting in the creation of surface tension gradients that drive the formation of Marangoni stresses. This work identifies the need to develop high-fidelity numerical models to study the influence of surfactants on the non-linear, three-dimensional physics of four industrially-relevant scenarios where thin films govern the dynamics.

This work uses a new massively-parallel solver for the simulation of three-dimensional thin film flows (Shin et al., 2018). The numerical technique adopts a hybrid front-tracking/level-set approach, making it extremely advantageous in the study of interfacial phenomenon in the presence of surfactants. Numerical models are developed for the study of elongated bubbles propagating through liquid capillaries. The presence of inertia in these systems is responsible for the formation of complex spatio-temporal undulating structures near the bubble tail. This work elucidates, for the first time, the effect of Marangoni stresses on the dynamics of these oscillations for a wide range of flow and surfactant-related parameters. Additionally, the work investigates the effect of surfactant addition on the thin film region of these bubbles and the development of %critical flow
vortical structures. The numerical approach is also extended to study the effect of Marangoni stresses on the three-dimensional wave dynamics of falling liquid films. Finally, the work concludes with the study of surfactant addition on the interaction of drops with thin liquid layers by looking at low-speed coalescence events first before arriving at high speed impacts. Escape from pinchoff was observed for all surfactant-laden coalescence systems, where a non-monotonic response was observed in relation to the vertical stretching and neck radius of the drop, driven by higher Marangoni stresses observed at the mid-range of the tested Marangoni parameters. Finally, surfactants were observed to significantly affect the vertical evolution of crowns in drop impact events, in addition to suppressing the breakup of the ejecta sheet for high Weber number impacts.