  We study the features of a radial Stokes flow due to a submerged jet directed
toward a liquid-air interface. The presence of surface-active impurities
confers to the interface an in-plane elasticity that resists the incident flow.
Both analytical and numerical calculations show that a minute amount of
surfactants is enough to profoundly alter the morphology of the flow. The
hydrodynamic response of the interface is affected as well, shifting from slip
to no-slip boundary condition as the surface compressibility decreases. We
argue that the competition between the divergent outward flow and the elastic
response of the interface may actually be used as a practical way to detect and
quantify a small amount of impurities.
