A liquid foam, usually consisting of large volumes of gas closely packed within thin films, possesses interesting mechanical properties that make it behave like solid and liquid, at rest and under shear, respectively. However, unlike solid foams which are widely used as mechanical dampers, the ability of liquid foams to absorb and dissipate energy, especially kinetic energy, is little known.

Anne Le Goff, ESPCI, Paris, France, and co-workers have quantified the kinetic energy absorbing process for liquid forms by monitoring solid balls being shot in foams from the Gillette shaving cream. The researchers observed two distinct deceleration steps for the projectiles, where the foam first reacted like a regular liquid before quickly turning elastic. They also developed a model to predict the amount of foam needed to completely stop the balls.

This study provides a new theoretical model to understand liquid foam rheology under high shear rates. Furthermore, the results may inspire the design and fabrication of novel bulletproof materials based on liquid foams.

• Shooting in a foam,
  Anne Le Goff, David Quéré, Christophe Clanet,
  Soft Matter 2014.
  DOI: 10.1039/C4SM00457D