As mentioned before, granular material can behave like a fluid, and in unexpected ways. For example, when a narrow tray of small spheres is shaken up and down, under certain conditions the spheres move up and down like a standing wave, as shown in the Caltech research group’s video and photo. Typically this fluidlike behavior appears at particular shaking frequencies and depths of particles.

Wave formation in a layer of shaken particles; the vibration of the tray is sinusoidal at 20 cycles per second; the particles are glass spheres about 1 mm in diameter.

In a similar experiment, first performed at the University of Texas at Austin and then at Argonne National Laboratory, researchers shook a much wider tray, resulting in the hexagonal pattern shown in the photo. Much the same kind of shaking happens during earthquakes, and it can produce both solid and fluid behavior, even at the same time. The theoretical analysis that explains these results promises to be widely applicable to the motion of many different kinds of granular materials.

One of the most surprising findings was the existence of individual oscillations, which in fact make up the hexagonal pattern above. The photo, taken at the University of Texas at Austin, shows a single peak and valley, surrounded by an undisturbed surface of the metal spheres.

These examples show how in the laboratory, as well as in the everyday world, granular materials display a wealth of interesting phenomena. The physicists who study them produce fundamental research into the nature of matter, and since these materials figure so prominently in our economy, this research may make possible important applications as well.

Dynamic Hexagon Patterns: Vertically vibrated sand organizes itself in a perfect honeycomb state. (photo courtesy of Materials Science Division, Argonne National Laboratory)