Rotating Waves of Grains

Snapshots of wavy rings of grains formed in a rotating cylinder filled with low-viscosity fluid and heavy particles.

Rotating drums are a popular way to explore granular dynamics. Here, researchers fill a cylinder (seen below) with heavy grains and a low-viscosity fluid, then rotate the mixture about a horizontal axis. This sets up a contest between centrifugal forces and gravitational forces on the grains. At the right rotation rates, the grains form annular rings around the outside of the cylinder, where they rotate at a different speed than the fluid. This difference in speed between the two layers can trigger a Kelvin-Helmholtz instability and cause waves along the interface between the grains and the fluid, as seen in the examples above. (Image and research credit: V. Dyakova and D. Polezhaev; top image adapted by N. Sharp)

Image of the experimental apparatus when not rotating.
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