  We measure the Marangoni elasticity of a flowing soap film to be 22 dyne/cm
irrespective of its width, thickness, flow speed, or the bulk soap
concentration. We perform this measurement by generating an oblique shock in
the soap film and measuring the shock angle, flow speed and thickness. We
postulate that the elasticity is constant because the film surface is crowded
with soap molecules. Our method allows non-destructive measurement of flowing
soap film elasticity, and the value 22 dyne/cm is likely applicable to other
similarly constructed flowing soap films.
