Thursday, October 06, 2005

The Truth About Quicksand Is Beginning to Sink In - New York Times

The Truth About Quicksand Is Beginning to Sink In - New York Times: "Real quicksand, the kind that is almost impossible to extricate yourself from, is not just water and sand. Salt and clay are also major ingredients in this B-movie plot device, scientists report in the current issue of Nature.

Their study began when Dr. Daniel Bonn, a professor of physics at the University of Amsterdam, was in Iran a few years ago and saw signs warning of quicksand. Naturally, the warning prompted him to collect samples and he sank in to his ankles.

He quickly escaped, but even shallow quicksand can be hard to step out of. Back home, Dr. Bonn and his colleagues found out why. Sand grains in quicksand are usually loosely packed, with the clay acting as a fragile gel holding the grains together.

Hit with sudden force from, say, a hapless victim, the quicksand gel turns to liquid. Then salt causes clay particles to stick to one another instead of the sand grains, with the result that a victim ends up surrounded by densely packed sand.

The force needed to pull out a person immersed in quicksand is about the same needed to lift a car, Dr. Bonn said. The trick for escaping is to slowly wiggle the feet and legs, allowing water to flow in. People float in quicksand so it is also impossible to sink all the way in, but quicksand usually forms at river estuaries, so a captive could drown at high tide."

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