Materials adsorbed to the surface of a fluid -- for instance, crude oil, biological slicks, or industrial/medical surfactants -- will move in response to surface waves. Our aim is to understand the spatiotemporal dynamics of this response.
Using non-invasive optical techniques for measuring both the surfactant distribution (fluorescence) and wave surface profile (Moire profilometry), we image the surfactant monolayer as it is advected by standing Faraday waves and traveling meniscus waves. We find that the surfactant shares the same pattern as the underlying waves but is temporally out of phase with the wave. The observed phase shifts place the surfactant in the trough of the standing Faraday wave and on the leading edge of the meniscus wave.
The spreading of surfactants on thin films is an industrially and medically important phenomenon, but the dynamics are highly nonlinear and visualization of the surfactant dynamics has been a long-standing experimental challenge. Our mathematics collaborators, Michael Shearer and Rachel Levy perform modelling and analysis of these same systems.
We perform quantitative measurements of the spreading of an insoluble surfactant on a thin layer of glycerin. During the spreading process, we directly observe both the radial height profile of the spreading droplet (horizontal line) and the spatial distribution of the fluorescently-tagged surfactant. We find that the spreading circular layer of surfactant forms a capillary ridge at its leading edge with a trough trailing the ridge. Both the capillary ridge and surfactant leading edge can be described to spread as R ~ t1/4 .