Marangoni Transport Synergism in Mixed Surfactant Systems

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Surfactants are compounds that lower the surface tension between two phases (for example, oil and water) and are important for a range of complex fluid products, such as laundry detergents, flooding fluids for enhanced oil recovery, and many food, pharmaceutical and consumer products. Fluid flows driven by gradients in surface tension are called "Marangoni flows" and have been well-studied in systems containing a single surfactant. However, industrially important products typically contain mixtures of surfactants. This research project considers how surfactant mixtures may produce strong cooperative or synergistic effects on these Marangoni flows. Experiments to measure these flows and the effects of the surfactants on the interfaces are being performed and compared to theories modified for these multi-component systems. While building on existing knowledge and methods, this research expands into fundamentally interesting and commercially important areas. Understanding the interactions between surfactants is a potentially powerful aid to product formulation across a host of industries. STEM human resource infrastructure is benefitting from the research training of graduate and undergraduate students. Additionally, middle school students are being mentored through the Carnegie Mellon University Physics Concepts Outreach Program in a project in which student-designed experiments reveal interfacial engineering concepts underlying liquid commercial product formulations.Marangoni synergism is the creation of stronger Marangoni transport in the presence of two surfactants than is obtained from either of the two on their own, at the same interfacial tension. This synergism results from the coupled effects of thermodynamics, diffusion, flow conditions, and kinetic rate constants on Marangoni transport. This research project demonstrates this synergism and identifies the conditions that produce it. Experiments are focusing on two-component surfactants in aqueous solutions flowing past a pure oil layer and single-surfactant aqueous solutions flowing past a surfactant-containing oil film. Transient interfacial velocities are being measured as the bulk composition is changed to drive the system out of equilibrium. These transport experiments are being complemented by equilibrium interfacial tension measurements and theoretical transport modeling to identify the influence of dynamic parameters on surfactant compositions that yield Marangoni synergism. The knowledge generated by this research fills a significant knowledge gap in the literature and enables new interfacial approaches for complex fluid formulation.