Objects inserted in a fast-flowing soap film are observed to cause strong disturbances on the film, resembling shock waves. Currently a prevailing opinion is that these shock waves correspond to sudden increases in film thickness caused by elastic symmetric waves, or the propagation of film thickness changes driven by surface tension variations with the film surface area. Here, we show by laser interferometry that these “shock waves” in fact originate from the antisymmetric waves studied by Taylor, which correspond to the bending of the liquid film with no thickness change. This phenomenon thus forms an example of symmetry breaking. These shock waves maintain an antisymmetric structure, but the structure size varies with the size of the intruding object. Our results indicate that these structures could be objects for studying solitary waves, rather than the traditional shocks corresponding to density jumps in compressible flows.

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• Capillary waves
• Flow instability
• Instability of free-surface flows
• Shock waves
• Thin fluid films