Abstract

When ruptured, a bursting soap film develops a thickened edge (“front”) that collects liquid. We see this in the viral “Slinky Bubble Trick” – a retracting soap film between the rungs of a helical slinky spring. However, unlike commonly studied, ideal infinite soap films that retract at the terminal Taylor-Culick speed, the slinky bubble exhibits rich transient phenomena. First, radial droplet ejection due to the centrifugal Rayleigh-Taylor instability of the front accelerates retraction, like how rocket exhaust propels spaceships forward. Then, friction due to viscous dissipation in the meniscus where the film meets the rungs slows retraction. Lastly, the thinning of the film under gravity causes its thickness to differ all around the slinky. This project develops a comprehensive, force-based theoretical model capturing all the above effects to predict the front’s time-dependent retraction speed along the rungs. Thin-film interference was used to measure the film thickness required for computations. Then, predictions are compared with extensive experiments using different slinky radii, rung spacing and orientation (horizontal or vertical), and fluid viscosities, achieving good agreement. Thinner, less viscous films retract faster, while rung spacing seems to have no effect. The proposed front instability is also corroborated. This investigation sheds light on the retraction of unstable thin films under real-world, confined conditions. It could help elucidate how bubbles on the ocean surface produce sea spray aerosol that play a key role in climate regulation through scattering solar radiation and seeding clouds.