The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Dynamic Wetting at Receding Contact Lines of Soluble Surfactant Solutions: Hydrodynamics and Molecular Assemblies

by Kalyani S. Varanasi



Almost any treatment of wetting in natural or technological settings must deal with fluids containing surface active materials. While previous work identified Marangoni flows and surfactant transport as important in wetting by surfactant solutions, it did little to quantify the impact of these phenomena on the dynamic wetting of soluble surfactant systems
In this thesis, we have experimentally investigated the hydrodynamics of receding contact lines and surfactant deposition from soluble surfactant solutions in two velocity regimes: at low substrate withdrawal speeds where the surface emerges dry, and at higher withdrawal speeds where a film is entrained on the surface. We have developed a technique to measure the fluid velocity on the liquid-vapor interface and this enables us to quantitatively study the Marangoni stresses and surfactant transport near the contact line. We determine the Marangoni stresses near the contact line. While they are the least dominant stresses, they do alter the flow fields compared to those of a pure fluid. The surfactant-surface interactions at the contact line govern the Marangoni stresses and thereby control the hydrodynamics. In the two velocity regimes examined, we observe two different mechanisms of surfactant deposition on the solid-vapor interface. We also see a sharp transition in the boundary between these two wetting regimes with surfactant concentration. We find that this transition is universal across several surfactant-surface systems.