In a recent paper whimsically titled “Elastic spheres can walk on water,” a group from Utah State University have empirically shown why it is easier to skip an elastic sphere (such as the water toy the WaBoBa) on water compared to a rigid sphere.
This kind of research does actually have a lot of practical implications beyond being a fun thing to do at the lake. Inflatable boats exhibit different behavior on water compared to a boat made of more rigid material, and it is important to understand why. Specifically, the Navy thinks it is important to understand why considering they were the ones who funded this research.
The physics of how a stone skips on water have been well-studied. Briefly, that phenomenon can be described as “an object obliquely impacting a water surface with sufficient inertia will carve a cavity on the air-water interface and experience a pressure-driven hydrodynamic force dependent on object velocity, geometry, and orientation.” That is a very science-y way of saying what you’ve probably figured out through trial-and-error: the stone needs to be kind of flat and thrown in the right way (more out, less down) and at the right speed to skip.
To gt a rigid sphere to skip (which is much harder than the favored flat stone), the impact angle on the water has to be below an upper limit, termed βo. An elastic sphere’s βo is up to three times larger than a rigid sphere’s so there is a much larger window of acceptable impact angles to get an elastic sphere to skip. This expanded βo (plus a lot of other intense mathematics) means that elastic spheres can do multiple skips over long periods of time.
In addition to tolerating higher impact angles, elastic spheres actually deform upon impact with water, giving them the more favorable disk shape that we all know works better for skipping rigid stones. In this slow-motion clip of an elastic sphere hitting the water, you can see how it flattens out on impact.
Hilariously, this line of research got started simply because the senior researchers’s son and nephew wanted to see a slow motion video of a rubber ball skipping on water. That curiosity led to important and translational discoveries on the way elastic objects interact with the surface of water.
Cassidy is a curious person, and her curiosity has led her to pursue a PhD in Pharmacology at the New York University Sackler Institute of Biomedical Sciences. She likes to talk about science way too much, so now she's going to try writing about it.
