Hole-Closing of a Surfactant Layer on a Thin Fluid Film

The spreading of surfactants on a thin fluid layer has been most commonly studied in an outward-spreading geometry. We perform simulations and experiments on the inverse, the inward spreading of surfactant into a clean disk-shaped region, known as hole-closing. In both cases, we observe that the inward force from the surface tension gradient produces a transient distention, in which the underlying fluid is raised within the closing region. We observe that the height of the distension is controlled by a combination of fluid depth and the surface tension gradient between the two regions. We compare the evolution of the distension height over time to a coupled system of partial differential equations that have been used to model surfactant spreading for more than two decades.