Spatiotemporal measurement of surfactant distribution on gravity–capillary waves

Abstract

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.

Impact of dissipation on the energy spectrum of experimental turbulence of gravity surface waves

“…A maximum is observed at about 4 Hz with a peak value close to 15. of the resonance was confirmed experimentally by Cini & Lombardini [3]. The variation of interfacial surfactant concentration due to the interaction of surface waves with the surfactant layer was directly observed by Strickland et al [17]. Note that dissipation occurs also through boundary layers at the bottom and at the vertical walls of the tank [18].…”

Three-dimensional dynamics of falling films in the presence of insoluble surfactants

“…This is a common assumption in the context of several previous falling liquid film studies (Blyth & Pozrikidis 2004;Wei 2005;Pereira & Kalliadasis 2008;Bhat & Samanta 2018;Hu et al 2020). Insoluble surfactants are also naturally occurring and we believe that the results from this paper could be of great interest to any experimental set-up that incorporates the use of insoluble surfactants such as NBD-PC (1-palmitoyl-2-{12-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl}-sn-glycero-3 -phosphocholine) (Strickland, Shearer & Daniels 2015;Fallest et al 2010). The gas and liquid are assumed to be immiscible, incompressible Newtonian fluids.…”

Mean flow produced by small-amplitude vibrations of a liquid bridge with its free surface covered with an insoluble surfactant

“…For instance, Martin and Vega [36] included the effect of Marangoni elasticity in a previous theory for clean free surfaces [32] to study the drift stability of standing Faraday waves in annular containers, previously found experimentally by Douady et al [37]. More recently, Strickland et al [38] have pointed out that materials (such as crude oil, biogenic slicks, or industrial and medical surfactants) absorbed at the fluid free surface are expected to move in response to surface waves. They have experimentally studied such effect for Faraday waves in a shallow cylindrical container with an insoluble surfactant monolayer.…”