In nature, objects like asteroids, black holes, and atomic nuclei can get distorted when spinning rapidly. Researchers are exploring these objects using a new model platform: particle rafts levitated by sound. The individual particles are less than a millimeter wide and tend to clump together due to the scattering of sound waves off neighboring particles. This effect provides a cohesive force — similar to surface tension or the effects of gravity — that draws the particles together. With the right frequency, the sound waves can also make the granular rafts spin, setting up a tug-of-war between cohesion and centrifugal force.
As the rafts spin, they distort, pull apart, and come back together. Interestingly, the cohesive force a raft experiences increases with the raft’s size. That makes the attractive force unlike surface tension (which is the same whether you have a bucket of water or a lake) and more like gravity (which is stronger with more material.) Because of this size dependence, the team hopes their granular rafts could be a new way to study the formation of rubble-pile asteroids and similarly granular systems.
(Video, image, and research credit: M. Lim et al.; via APS Physics)