78th Annual Meeting of the APS Division of Fluid Dynamics (Nov 23 — 25, 2025)

V012: Spreading and Dewetting of Water-Isopropanol Drops on Surfactant-laden Oil Substrate

When a drop of a binary mixture of water and isopropanol is placed on a substrate of mineral oil, it will spread and form a thin film if the spreading parameter of the system is positive. Over time, increased evaporation of isopropanol at the edge of the drop creates a surface tension gradient, which causes droplets to be ejected from the drop perimeter. This phenomenon is known as Marangoni bursting (Keiser et al. 2017). When surfactant is dissolved into the oil layer, we encounter several surprising and delightful spreading and dewetting behaviors. At small concentrations (near the critical micelle concentration), adding surfactant decreases the droplet size. The surfactant renders the ejected droplets prone to 'popping.' When they pop, the droplets suddenly spread, each forming a disk with a thickened rim, then retract. However, with the addition of a bit more surfactant, spreading is inhibited, and Marangoni bursting is suppressed. When surfactant concentration is further increased, drop spreading resumes. Extremely thin films are created, which leave behind web-like or lace-like patterns of water when the isopropanol evaporates.