Spreading and Dewetting of Alcohol-Water Drops on a Surfactant-Laden Oil Layer

When a drop of a binary mixture of water and isopropanol is placed on a substrate of 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 and causes the film to recoil and dewet the surface of the oil layer while ejecting smaller droplets from its perimeter. This phenomenon is known as Marangoni bursting (Keiser et al. 2017). We experimentally investigate how the spreading and dewetting of the drop is affected by the presence of surfactant dissolved in the oil. Laser shadowgraphy is used to visualize water-alcohol drops spreading on a layer of mineral oil that is laced with Span 80, an oil-soluble surfactant. We report observations and measurements for a range of isopropanol and surfactant concentrations. As the surfactant concentration is increased, we observe a transition from the well-known bursting behavior that leaves behind a radial array of droplets to a mode of dewetting that leaves a thin film with a web--like structure.