Shockwave / foam film interaction*

Foams are very efficient to absorb shock waves, and are used in demining situations. The wave/film interaction involves complex processes (Bremond & Villermaux, J. Fluid. Mech. 2005) not fully understood yet. To elucidate the various processes leading to film rupture under shock, we produce foam films in a transparent shock tube and image them at 200000 fps, by direct visualization or ombroscopy. Deformations at a millimetric scale are observed before the film eventually ruptures, in which the film thickness heterogeneities play a crucial role : pieces of film of different thicknesses are accelerated at various rates, leading to film tear out, and only the most homogeneous films have time to destabilize via the Rayleigh Taylor instability described in the literature. The orientation of the film with respect of the incoming wave also influences the film deformation and, using dedicated film structure, we show that the tangential component of the acceleration generates wrinkling of well defined wave length. The pressure signals induced by the shock / film interaction are in good agreement with numerical simulations and show the short delay generated by the film, contributing to the wave attenuation.