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 report experiments on the inverse, the inward spreading of surfactant into a clean disk-shaped region, known as hole-closing. We observe that the inward force produces a transient distention, in which the underlying fluid is raised within the closing region. Using a laser line to image the height profile of the fluid surface, we characterize the height and evolution of the fluid distention. We observe that the height of the distension is controlled by a combination of fluid depth, surface tension difference, and chemical composition of both fluid and surfactant. Once formed, the height of the distension decays approximately exponentially, with the timescale primarily set by the particular choice of surfactant and underlying fluid.