When an object hits water whether it draws air in with it depends on its shape, impact speed, and surface characteristics. In this experiment, though, there’s a bit of a twist. Here the sphere is passing through an interface with surfactants added. On the left, the sphere passes through smoothly without entraining air or creating a cavity. On the right, the same sphere impacts at the same speed but this time the interface is covered in a layer of bubbles. As a result, the sphere pulls a large air cavity into the water with it. Why the big difference?

As the sphere passes through the bubbles, they burst, spraying the sphere with droplets. On impact, those droplets disrupt the layer of water traveling up the sides of the sphere, causing it to pull away from the surface and form a splash. Instead of smoothly coating the sphere in water, air can now stick to the sphere and get pulled in with it. (Image and research credit: N. Speirs et al., source)

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