Magazine article Science News

Trickling Grains, Sandpiles, and Avalanches

Magazine article Science News

Trickling Grains, Sandpiles, and Avalanches

Article excerpt

The slow trickle of sand through an hourglass produces more than just a neat, conical sandpile. Close observation reveals a rich array of phenomena, among them the occurrence of miniature avalanches triggered by newly fallen grains and the seemingly automatic settling of particles into a pile with a characteristic slope.

To help illuminate the details of sandpile formation, physicist Franco M. Nori and his co-workers at the University of Michigan in Ann Arbor devised an apparatus that considerably reduces the speed at which grains of sand arrive at a pile's surface. Such a deposition strategy allows the researchers to concentrate on the subtle rearrangements and tiny avalanches that occur when grains arrive at very low speeds.

The apparatus they use to demonstrate "two-dimensional" sandpile formation consists of a strip of sand at the top, a layer of air bubbles in the middle, and a liquid at the bottom, all sandwiched between two parallel, vertical glass plates. The air bubbles act as a valve, trapping the sand so that only a few grains at a time can escape through a small gap among the bubbles into the liquid layer. The released grains then drift slowly down through the liquid and gradually build up into a sandpile.

As shown in the photograph, the deposited sand grains (of two different colors) rearrange themselves and settle so that the angle of the sandpile slope maintains a characteristic value.

To track avalanches, Nori's group used another apparatus, consisting of a high-walled acrylic tray containing a thick layer of tiny beads, that operates somewhat like the back of a dump truck. …

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