Beware! Sand in This Physics Lab May Eat You Alive
By KENNETH CHANG
Published: December 14, 2004
Beware of playing in any sandboxes you might find in the laboratory of Dr. Detlef Lohse.
Traditional deathtrap quicksand is a slurry of sand, water and clay. The water keeps the sand from sticking together to support weight, and a person who steps in slowly sinks.
Now Dr. Lohse, a professor of applied physics, and his colleagues at the University of Twente in the Netherlands show that it is possible to vanish into a pile of completely dry sand as well. Worse, their sand looks the same as the normal, weight-supporting variety.
In the current issue of the journal Nature, the researchers describe their experiment. They puffed air through fine-grained sand, shuffling the grains of sand into a more precarious arrangement -- something like a house of cards. Even after the air was turned off and the sand settled, the packing density of the grains was an airy 41 percent, down from 60 percent initially.
The researchers suspended a Ping-Pong ball, weighed down with bits of bronze, so that it just touched the surface of the sand. They then burned the string holding the doomed ball. The fluffy sand was incapable of supporting the ball, which immediately and quickly sank without a splash.
A tenth of a second later, the backwash of sand closing up behind the sinking ball caused a thin narrow jet of sand to shoot several inches into the air, like a desperate distress call. Heavier balls sank deeper into the sand, and the jets occurred only for those weighing more than an ounce.
Dr. Lohse said the findings could explain reports of travelers' being swallowed up in the desert.
''The U.S. Army is very interested in this,'' he said, ''because these days, the U.S. Army tends to go to desert states.''
Photos: Snapshots show a weighted Ping-Pong ball sinking into dry quicksand. The 4.7-ounce ball disappears in about one-tenth of a second and then expels a narrow jet of sand. (Photos Courtesy of Dr. Detlef Lohse/Nature)