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The air bubble entrapped under a drop impacting on a solid surface
Published online by Cambridge University Press: 02 December 2005
Abstract
We present experimental observations of the disk of air caught under a drop impacting onto a solid surface. By imaging the impact through an acrylic plate with an ultra-high-speed video camera, we can follow the evolution of the air disk as it contracts into a bubble under the centre of the drop. The initial size and contraction speed of the disk were measured for a range of impact Weber and Reynolds numbers. The size of the initial disk is related to the bottom curvature of the drop at the initial contact, as measured in free-fall. The initial contact often leaves behind a ring of micro-bubbles, marking its location. The air disk contracts at a speed comparable to the corresponding air disks caught under a drop impacting onto a liquid surface. This speed also seems independent of the wettability of the liquid, which only affects the azimuthal shape of the contact line. For some impact conditions, the dynamics of the contraction leaves a small droplet at the centre of the bubble. This arises from a capillary wave propagating from the edges of the contracting disk towards the centre. As the wave converges its amplitude grows until it touches the solid substrate, thereby pinching off the micro-droplet at the plate, in the centre of the bubble. The effect of increasing liquid viscosity is to slow down the contraction speed and to produce a more irregular contact line leaving more micro-bubbles along the initial ring.
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- © 2005 Cambridge University Press
Thoroddsen et al. supplementary movie
An ultra-high-speed video clip, showing the entrapment of an air bubble under an impacting water drop. The drop release height is H=3.55 cm, giving We=34 and Re=2800. As the air sheet contracts capillary waves propagate towards the centre, where they touch the plate and pinch off a small droplet inside the bubble. The horizontal extent of the frame is 3.51 mm and the numbers show the time in micro-seconds.
Thoroddsen et al. supplementary movie
An ultra-high-speed video clip, showing the entrapment of an air bubble under an impacting water drop. The drop release height is H=3.55 cm, giving We=34 and Re=2800. As the air sheet contracts capillary waves propagate towards the centre, where they touch the plate and pinch off a small droplet inside the bubble. The horizontal extent of the frame is 3.51 mm and the numbers show the time in micro-seconds.
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