Data from: Spatiotemporal measurement of surfactant distribution on gravity–capillary waves

Materials adsorbed onto the surface of a fluid – for instance, crude oil,
biogenic slicks or industrial/medical surfactants – will move in response
to surface waves. Owing to the difficulty of non-invasive measurement of
the spatial distribution of a molecular monolayer, little is known about
the dynamics that couple the surface waves and the evolving density field.
Here, we report measurements of the spatiotemporal dynamics of the density
field of an insoluble surfactant driven by gravity–capillary waves in a
shallow cylindrical container. Standing Faraday waves and travelling waves
generated by the meniscus are superimposed to create a non-trivial
surfactant density field. We measure both the height field of the surface
using moiré imaging, and the density field of the surfactant via the
fluorescence of NBD-tagged phosphatidylcholine, a lipid. Through phase
averaging stroboscopically acquired images of the density field, we
determine that the surfactant accumulates on the leading edge of the
travelling meniscus waves and in the troughs of the standing Faraday
waves. We fit the spatiotemporal variations in the two fields using an
ansatz consisting of a superposition of Bessel functions, and report
measurements of the wavenumbers and energy damping factors associated with
the meniscus and Faraday waves, as well as the spatial and temporal phase
shifts between them. While these measurements are largely consistent for
both types of waves and both fields, it is notable that the damping
factors for height and surfactant in the meniscus waves do not agree. This
raises the possibility that there is a contribution from longitudinal
waves in addition to the gravity–capillary waves.