  We investigate the dynamics of an insoluble surfactant on the surface of a
thin viscous fluid spreading inward to fill a surfactant-free region. During
the initial stages of surfactant self-healing, Marangoni forces drive an
axisymmetric ridge inward to coalesce into a growing central distension; this
is unlike outward-spreading, in which the ridge decays. In later dynamics, the
distension slowly decays and the surfactant concentration equilibrates. We
present results from experiments in which we simultaneously measure the
surfactant concentration (using fluorescently-tagged lipids) and the fluid
height profile (via laser profilometry). We compare the results to simulations
of a mathematical model using parameters from our experiments. For surfactant
concentrations close to but below the critical monolayer concentration, we
observe agreement between the height profiles in the numerical simulations and
the experiment, but disagreement in the surfactant distribution. In experiments
at lower concentrations, the surfactant spreading and formation of a Marangoni
ridge are no longer present, and a persistent lipid-free region remains. This
observation, which is not captured by the simulations, has undesirable
implications for applications where uniform coverage is advantageous. Finally,
we probe the generality of the effect, and find that distensions of similar
size are produced independent of initial fluid thickness, size of initial clean
region, and surfactant type.
