  It is widely thought that in steady, gravity-driven, unobstructed soap-film
flows, the velocity increases monotonically downstream. Here we show
experimentally that the velocity increases, peaks, drops abruptly, then lessens
gradually downstream. We argue theoretically and verify experimentally that the
abrupt drop in velocity corresponds to a Marangoni shock, a type of shock
related to the elasticity of the film. Marangoni shocks induce locally intense
turbulent fluctuations and may help elucidate the mechanisms that produce
two-dimensional turbulence away from boundaries.
