Droplet spreading on a thin viscous film

Abstract

We investigated experimentally the flows induced by a localized surfactant (oleic acid) on thin glycerol films. The oleic acid creates surface-tension gradients, which drive convention on the surface and within the film. Qualitative descriptions of the Lagrangian flow field were provided by flow-visualization experiments. Quantitative measurements of surface flows were conducted using dyed glycerol markers, where the initial motion of these markers is used to define the position of the time-dependent ‘convection front’. The flow characteristics were found to depend largely upon the magnitude of a gravitational parameter, G, representing the ratio of gravitational to surface-tension gradient (Marangoni) forces. Small G (G < 0.5) caused net outflow of the film leading to this thinning and, in some cases, to film rupture. When G < 1, bi-directional flows were caused by hydrostatic pressure gradients which served to stabilize the film. Additionally, the position of a surface convection front was found to differ significantly from that of the surfactant's leading edge for all G > 0. For this reason, surface markers may not be used to measure accurately the position of the droplet's leading edge. Finally, simulations of the Lagrangian flows conducted using the theory of Gaver & Grotberg (1990) compare favourably with these experimental results in the limit of dilute surfactant concentrations, and thus experimental verification of that theory is provided by this work. The results of this study may be useful for understanding the behaviour of the lung's thin-film lining after an aerosol droplet of insoluble exogenous surfactant lands upon its surface.