Impact, puncturing, and the self-healing of soap films

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Original entry: Pratomo Putra (Tom) Alimsijah, APPHY 226, Spring 2009

Impact, puncturing, and the self-healing of soap films

Laurent Courbin and Howard A. Stone, Phys. Fluids, 18, 091105, 2006


Soft Matter Keywords

Surface tension, elasticity


Abstract

The authors experimented with releasing liquid droplets and rigid spheres from a specific height onto soap films. The authors found that in the case of liquid droplets falling from a small enough heights, they deformed the soap film and bounced back up. However, from large heights, when the deformation of the film is too large, the droplet continued to the ground while the film returned to its initial state. It also healed itself when a table-tennis ball passes through it! This is surprising since the critical size for opening a hole in soap film without breaking it is expected to be of the order of the film thickness. The film is obviously order of magnitudes thinner than the tennis ball! It took six films to stop the tennis ball.


Soft Matter Discussion

When the tennis ball hit the film, energy is transferred to the material during impact. This energy is transferred either elastically (stored) or inelastically (friction, heat). If the impact energy is not that large, the energy will be transferred back as the object potential energy, thus the object bounces up. This is caused by the surface tension of the film. One example is shown here:


Fig. 1: A liquid droplet impacting a soap film and bouncing back up


However, when the impact is large enough, the tennis ball deformed the film before going through the film. The film thus returned to its initial state. This is shown below:


Fig. 2


It is interesting, since the critical size for opening a hole in a soap film without breaking it is expected to be of the order of the thickness of the film.


I will argue that this phenomenon is caused by the elasticity of the film, elongating it before snapping it back to its original state. In the tennis ball case, the elasticity of the film stores energy in the inward and up direction thus the film appears to self-heal.