  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.
