Shaken bead beds show pimples and dimples.

From a zebra's stripes or a leopard's spots to expanses of evenly spaced sand dunes, pattern formation is a common natural phenomenon.

Scientists can make regular patterns in the laboratory by vibrating thin beds of granular materials such as sand, sugar, or tiny beads. Shaken up and down, the surfaces of these materials develop ridges and hollows that form arrays of stripes, squares, or hexagons.

These large-scale arrangements result from the banging together of neighboring particles in the material. Now, researchers have discovered that the same interactions can lead to the formation of individual, local features that resemble splashes in a puddle of water. Unlike water splashes, however, these structures don't spread. Instead, they slosh back and forth between a craterlike and a peaked geometry in time with the vertical vibrations.

Paul B. Umbanhowar of the University of Texas at Austin and his coworkers report this finding in the Aug. 29 Nature.

The researchers performed their experiments using bronze spheres, each about 0.15 millimeter in diameter, spread in a layer across the bottom of a wide cylinder in a vacuum.

When the layers were less than 15 particles deep, the researchers observed reproducible geometric surface patterns determined by the rate and amplitude of vibration. Thicker layers usually remained flat and featureless until the vibrated system was momentarily disturbed, then one or more isolated structures emerged.

Called oscillons, these features show up as a peak during one vibrational cycle and a crater during the next. They are remarkably robust, persisting for more than 500,000 container oscillations.

Oscillons slowly drift across the surface. When oscillons in opposite phase approach each other, they attract, partially overlap, and form a pair that moves around as a unit. Oscillons in the same phase tend to avoid each other.

Precisely why oscillons emerge and how they maintain their form is not yet fully understood, Umbanhowar says. Further studies of these features may shed light on the nature of granular flow and aid in the development of improved methods of mixing, sorting, and handling granular materials.