The Sciences

The Year in Science: Physics 1997

Simple as Sand

By Robert PoolJan 1, 1998 6:00 AM


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Most reports in scientific journals describe discoveries that the average person has to take on faith. The research demands expensive equipment, specialized expertise, and years of work. But in March physicists published a surprising finding—an explanation for how sand arranges itself in layers on its way to becoming sandstone—that would cost only a few dollars and take only a few hours to replicate.

Go to a pet store and buy some nice sand, instructs Eugene Stanley of Boston University, describing the experiment done by his graduate student Hernán Makse. Then go to the supermarket and get some colored sugar. Take roughly equal parts of each and shake them together. Next proceed to your local hardware megastore for two nine-inch squares of Plexiglas and from them build a tall, thin container like those used for ant farms. You can use a few pennies as spacers and clamp the sheets of Plexiglas together with alligator clips. Tape off three sides, leaving only the top open, and you’re ready to go.

Put the mixture into an empty mustard bottle and slowly pour it into a heap on one side of the box, Stanley says. What happens as you pour is kind of a miracle. The sand and sugar, once thoroughly mixed, separate themselves into layers: sand, sugar, sand, sugar, and so on, all the way to the top of the pile.

Here’s how it works: As the mixture is poured into a heap, it mounds higher and higher. Every so often the mound reaches a critical angle, at which point the grains avalanche down the sides. As they reach the bottom of the heap, they form a little kink. The smaller, smoother grains of sand stop short at the bottom of the kink, while the bigger, rougher grains of sugar go just a bit farther, riding over the sand. As more grains tumble down the heap, the kink rides upward, forming two layers as it goes. Each succeeding avalanche contributes another pair of layers.

Stanley and Makse believe this observation explains how sand grains of different sizes and textures form layers in natural sandstone. But the phenomenon had never been demonstrated until Makse did it for his Ph.D thesis. No one had done such an idealized version of the experiment, Stanley says. Perhaps in this era of multimillion-dollar research laboratories it was just too simple.

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