A fluid counterpart to a solid string can be created by flowing liquid under gravity between vertical walls. Transverse and longitudinal capillary waves will propagate along the resulting liquid filament, and we recently discovered a triadic interaction leading to pattern formation. During the internship we would like to explore how the presence of surfactants modifies the patterns so far observed for a pure newtonian liquid. As the instability can be triggered in a large range of forcing frequencies, this would provide a way to probe the physical and rheological properties of surfactants at tunable time and spatial scales.