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Nat Commun. 2016 Feb 4;7:10551. doi: 10.1038/ncomms10551.

Elastic spheres can walk on water.

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Naval Undersea Warfare Center Division Newport, 1176 Howell Street, Newport, Rhode Island 02841, USA.
Department of Mechanical and Aerospace Engineering, Utah State University, ENGR 419J, Logan, Utah 84322, USA.
School of Engineering, Brown University, 184 Hope Street, Providence, Rhode Island 02912, USA.

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Incited by public fascination and engineering application, water-skipping of rigid stones and spheres has received considerable study. While these objects can be coaxed to ricochet, elastic spheres demonstrate superior water-skipping ability, but little is known about the effect of large material compliance on water impact physics. Here we show that upon water impact, very compliant spheres naturally assume a disk-like geometry and dynamic orientation that are favourable for water-skipping. Experiments and numerical modelling reveal that the initial spherical shape evolves as elastic waves propagate through the material. We find that the skipping dynamics are governed by the wave propagation speed and by the ratio of material shear modulus to hydrodynamic pressure. With these insights, we explain why softer spheres skip more easily than stiffer ones. Our results advance understanding of fluid-elastic body interaction during water impact, which could benefit inflatable craft modelling and, more playfully, design of elastic aquatic toys.

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