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Creation and rupture of thin liquid films : the generation of floating and interfacial bubbles

Laurent Courbin Institut de Physique de Rennes, UMR CNRS 6251

Thin liquid films, such as soap films and bubbles, have been studied extensively over the centuries as they impact a wide range of phenomena, in physics, chemistry, and engineering.
Here, we discuss the formation of soap bubbles starting with floating bubbles made by blowing air through a nozzle onto a soap film. We work either with circular bubble wands or vertically-falling soap films. We vary film size and thickness, nozzle radius, space between the film and nozzle, and gas density, and we measure the critical air speed above which bubbles are formed. The response is sensitive to confinement, i.e., the ratio between film and nozzle sizes, and dissipation in the turbulent gas jet which is a function of the distance between film and nozzle. We rationalize the observed four different hydrodynamic regimes by comparing the dynamic pressure exerted by the jet on the film and the Laplace pressure needed to create the curved surface of a bubble.We then study the generation of interfacial bubbles − bubbles on a liquid-gas or solid-gas interface – often seen on the ground after a storm and involved in the production of aerosol droplets across the oceans when ruptured. First, we investigate the formation of such bubbles when withdrawing a circular wire ring from a bath of soap solution, ring and liquid-gas interface being parallel to each other. The process creates an axisymmetric soap film that becomes unstable above a critical height and collapses to leave a planar film on the ring and an interfacial bubble in contact with the bath. Second, we study the creation of such bubbles when the ring that holds the planar film is moved back into the pool of liquid. Observations show that the film deforms upward as the ring becomes close enough to the liquid-gas interface, and the entry of the ring in the liquid results in the formation of an interfacial bubble. For both experimental configurations, we measure the size of the produced bubbles as a function of the ring size, the speed at which the ring moves, and the fluid properties, and we rationalize these experiments using simple physical arguments.