Micro-bubbles caught under impacting drops

Abstract

We use ultra-high-speed video imaging to study the formation of micro-bubbles under a drop impacting onto a pool surface. When the drop impacts at low velocities, an air layer cushions the impact and prevents immediate contact with the pool. This air layer can stretch into a hemispheric shape which area can become larger than the drop surface, but is locally thinner than a micron. We use the recent discovery of Saylor and Bounds1 that these films are more stable for silicone oils than water, which allows us to study the details of the breakup of the air film when it approaches 100 nm thickness. We observe a variety of breakup mechanisms which are imprinted onto the microbubble morphology. In Fig. 1 we show two different breakups, for different viscosity liquids. In Fig. 1(a) we show a ring of isolated holes appearing a fixed depth, where the film is thinnest. In part (b) we show a net of bubbles which form when the first rupture occurs at the bottom tip of the air hemisphere, leading to axisymmetric breakup as the rupture moves up along the sides to the original pool surface. The rupture at the bottom tip is promoted by the deformations of the drop, during the impact. The top of the drop engulfs an air cylinder which produces a jet which penetrates down through the drop to puncture the air film. Isolated ruptures lead to the formation of bubble necklaces, whereas a ring of ruptures form bubble chandeliers. When the impact Weber number is increased, the depth of the ruptures moves closer to the pool surface approaching the entrapment of air disks observed in earlier studies.2 We measure the speed of the breakup edge and find that for very viscous liquids, the breakup moves faster than the capillary-viscous velocity, which presumably occurs through repeated ruptures. This system may prove ideal for repeatably studying van der Waals interactions for extremely thin films of gas, within a liquid.