  The structure and stability of the convective flow generated by a source
located at the water surface containing an insoluble surfactant layer are
experimentally investigated. Application of a few types of source, which differ
in the force driving the interface, and two surfactants with different
rheological properties made it possible to generalize the results and to
develop a unified approach to describing the flow patterns observed in this
study. It is shown that, regardless of source type and surfactant used, the
problem under consideration can be completely described in terms of two
non-dimensional parameters. The first is the elasticity number introduced
earlier by Homsy and Meiburg (J. Fluid Mech., v.139, 1984). This parameter is
the ratio of two shear stresses formed at the interface by surfactant and
source, and it defines the formation of the convective zone at the center
nearby the source and the size of the stagnant zone at the periphery. The
second, called the surface Rayleigh number, is introduced for the first time in
this paper. It is responsible for the appearance of the multi-vortex flow
structure within the stagnant zone. We suppose that this structure occurs as a
result of the mechanical equilibrium instability of the surfactant layer under
the action of the momentum flux transported from the subphase by the viscous
mechanism. Based on our results and those reported earlier by other
researchers, we have determined the critical value of the surface Rayleigh
number and analyzed the possibility and conditions for the occurrence of this
instability, called by us the shear-driven surfactant layer (SDSL) instability,
by addressing some problems of interfacial hydrodynamics.
