Puncturing a drop using surfactants

Abstract

We present experiments, using an ultra-high-speed video camera, which indicate flow-induced buckling of a surfactant layer on the free surface of a deforming water drop. This results in the ‘puncturing’ of the liquid surface, sending out a narrow jet. The phenomenon is produced when a water drop falls onto and passes through a hemispherical soap film. The soap film wraps around the drop forming a closed pocket, with a thin layer of air separating the two. This occurs without initial contact between the soap film and the drop liquid, which becomes clear during the subsequent dynamics, as the original film unwraps fully from the drop surface. Under particular conditions the soap film makes contact with the top of the drop, producing a toroidal bubble, which is driven down the drop to form a hemisphere at its bottom. We propose that the resulting rapid reduction in surface area of the bubble leads to overpacking of the surfactant molecules and buckling of the surface, which allows the jet to emerge, from the bottom tip of the drop. Marangoni-driven boundary layers, converging at the bottom, may also play a role in forcing out the jet.