How spheres impact water has been studied for more than a century. The typical impact for a rigid sphere creates a cavity like the one on the upper left – relatively narrow and prone to pinching off at its skinny waist. If the sphere is elastic –squishy – instead, the cavity ends up looking much different. This is shown in the upper right image, taken with an elastic ball and otherwise identical conditions to the upper left image. The elastic ball deforms; it flattens as it hits the surface, creating a wider cavity. If you watch the animations in the bottom row, you can see the sphere oscillating after impact. Those changes in shape form a second cavity inside the first one. It’s this smaller second cavity that pinches off and sends a liquid jet back up to the collapsing splash curtain.
From the top image, we can also see that the elastic sphere slows down more quickly after impact. This makes sense because part of its kinetic energy at impact has gone into the sphere’s shape changes and their interaction with the surrounding water.
If you’d like to see more splashy stuff, be sure to check out my webcast with a couple of this paper’s authors. (Image credits: top row – C. Mabey; bottom row – R. Hurd et al., source; research credit: R. Hurd et al.)
