In-situ observation of collective bubble collapse dynamics in a quasi-two-dimensional foam

Abstract The stability of foams is an important subject not only for fundamental science, but for applications in daily life. The most destructive phenomenon underpinning foam collapse is a collective bubble collapse, yet the mechanism behind this is unclear. In this study, we clarify the dynamics of the collective bubble collapse in a quasi-two-dimensional foam by in-situ observation with a high speed camera. We find two modes for collective bubble collapse: one is the propagation of liquid film breakage via impact with the stream of another broken liquid film. The other is breakage of a distant liquid film due to penetration by a liquid droplet, emitted by impact with the flow of a broken liquid film. As the liquid fraction increases, the velocity of liquid droplets decreases. Instead of penetration, the liquid droplet bounces like a billiard ball or it is absorbed into other films

Leidenfrost on a ratchet

International audienceAs discovered by Leidenfrost, liquids placed on very hot solids levitate on a cushion of their own vapour. These model hovercrafts are remarkably mobile: placed on a hot ratchet, a droplet not only levitates, but also self-propels, in a well-defined direction, at a well-defined velocity (typically, 10 cm s -1). The challenge is to understand the origin of the phenomenon, which contrasts with other situations where an asymmetry in the solid/liquid contact was used to generate liquid self-propulsion. We consider Leidenfrost solids that directly sublimate on hot substrates, and show that they also self-propel on ratchets. This leads to a scenario for the motion: the vapour flow escaping below the Leidenfrost body gets rectified by the presence of asymmetric textures, so that a directional thrust drives the levitating material. Using fishing lines to catch drops, we measure the force acting on them, and discuss both the driving force and the special friction generated by the textures