Soap Films Burst Like Flapping Flags

When punctured, a flat soap film bursts by opening a hole driven by liquid surface tension. The hole rim does not, however, remain smooth but soon develops indentations at the tip of which ligaments form, ultimately breaking and leaving the initially connex film into a mist of disjointed drops. We report on original observations showing that these indentations result from a flaglike instability between the film and the surrounding atmosphere inducing an oscillatory motion out of its plane. Just like a flag edge flaps in the wind, the film is successively accelerated on both sides perpendicularly to its plane, inducing film thickness modulations and centrifuging liquid ligaments that finally pinch off to form the observed spray. This effect exemplifies how the dynamics of fragile objects such as thin liquid films is sensitive to their embedding medium.

Figure 1

Destabilization of a soap film receding edge. Left: Sequence of the opening of a soap film punctured by the projection of a spark. In the two first images, the film edge is smooth. Growing corrugations appear in the third image and reach a saturated size. Indentations (or fingers) obvious in the last image are ligaments draining the liquid and continually pinching off to form the resulting mist. Images are sampled every 6.7 ms. Right: Snapshot emphasizing on indentations. Note the arabesques, ligaments detaching from the rim as they flap like a whip.

Figure 2

Flapping motion. Main: Sequence of a flapping receding soap film from a lateral view. A laser sheet under normal incidence lights the fluorescent film emphasizing the out of plane motions. The initially flat film is punctured on the left-hand side and, as it recedes, initiates a characteristic flapping motion close to its edge which grows in amplitude. Images are sampled every 2.3 ms. Inset: Flapping pulsation $\omega$ versus ${\rho }_a{V}^3/{\sigma }_0$, where ${\rho }_a$ is the outer medium density. The solid line is the expected scaling law in Eq. (1).

Figure 3

Influence of the outer medium density. Inset: Delay time ${\tau }_d$ before rim destabilization versus receding velocity $V$ for films bursting in air and in sulfur hexafluoride (${\mathrm{SF}}_6$). The dotted lines have a slope $-3$. Main: Delay ${\tau }_d$ versus ${\sigma }_0/{\rho }_a{V}^3$; the solid line is the expected scaling law.

Figure 4

Indentation size. Indentation transverse wavelength ${\lambda }_{\perp }$ as defined in the inset versus ${\sigma }_0/{\rho }_a{V}^2$. The solid line is the expected scaling law.