Surfactant-driven instability of a divergent flow

TL;DR

This paper investigates how tiny amounts of surfactants cause instability in divergent water flows, revealing a surfactant-driven vortex formation in a shear response experiment supported by simulations.

Contribution

It introduces a novel radial flow experiment to study surfactant effects and identifies a surfactant-driven instability occurring in the Stokes regime, supported by numerical simulations.

Findings

Surfactants induce a vortex pair in divergent flows.

Instability occurs in the inertia-less Stokes regime.

Supported by numerical simulations.

Abstract

Extremely small amounts of surface-active contaminants are known to drastically modify the hydrodynamic response of the water-air interface. Surfactant concentrations as low as a few thousand molecules per square micron are sufficient to eventually induce complete stiffening. In order to probe the shear response of a water-air interface, we design a radial flow experiment that consists in an upward water jet directed to the interface. We observe that the standard no-slip effect is often circumvented by an azimuthal instability with the occurence of a vortex pair. Supported by numerical simulations, we highlight that the instability occurs in the (inertia-less) Stokes regime and is driven by surfactant advection by the flow. The latter mechanism is suggested as a general feature in a wide variety of reported and yet unexplained observations.