Granular materials such as sand may slow high-speed projectiles by turning kinetic energy from an incoming object into complex networks of sound waves.
Abram Clark at Duke University in Durham, North Carolina, and his colleagues studied the impact of bronze disks falling onto a bed of photoelastic grains — plastic particles that light up under pressure. Filming at 40,000 frames per second, the team found that falling disks pulsed acoustic energy along branching networks of grains that glowed as pressure travelled through the material (pictured). The 'force chains' decayed exponentially and died out after passing through about 10 particles. Falling disks decelerated in spurts that matched closely in time with acoustic fluctuations through the grain bed, suggesting that acoustic waves play a key part in dissipating the energy of projectiles that strike granular materials.
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Grains call a halt with sound. Nature 492, 315 (2012). https://doi.org/10.1038/492315b
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DOI: https://doi.org/10.1038/492315b