Marangoni flows induced by soluble surfactants
In a nutshell
A solution of soluble surfactants is injected into pure water. The surface flows observed are a macroscopic manifestation of the solution’s surface rheology properties.
A few details
The injected solution contains surfactants and small drops of oil. The latter allow us to visualize the flow by reflecting the light (the experiment is illuminated from above).
The concentration of surfactants is high at the point of injection, and decreases with distance. This induces a surface tension gradient, giving rise to a surface flow. This is known as the Marangoni effect. The black disk we see corresponds to this surface flow. The lower the solubility of the surfactants, the wider the disk. At the edges of the disk, flow stoppage leads to the generation of surface vortex pairs, arranged in a regular pattern along the flow contour.
The flow observed is highly dependent on surfactant properties, and this experiment could serve as a quality control tool when looking for biobased alternatives to petrochemical surfactants.
References
about the black disk: M Roché, Z Li, IM Griffiths, S Le Roux, I Cantat, A Saint-Jalmes, H A Stone, “Marangoni flow of soluble amphiphiles”, Physical review letters (2014)
about the vortex: Gabriel Le Doudic (2022), “Écoulements solutocapillaires en présence d’échange interface-volume : génération de vorticité interfaciale et propulsion”
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