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Sinking feeling. Taz, who has approximately the same density as a human body, sinks only halfway in a quicksand system stressed by shaking.

Diving Deep Into Quicksand

Quicksand has the unsettling property of being easier to get into than out of. Now a new analysis of this treacherous mixture of earth and water may explain why, as well as provide a few survival tips to anyone unlucky enough to get trapped.

On a recent vacation to Iran, Daniel Bonn of the University of Amsterdam in the Netherlands and the École Normale Supérieure in Paris came across a warning sign for quicksand next to a salt lake. Being a specialist in granular materials, Bonn brought a sample back to his lab to decipher what makes the stuff so dangerous.

Bonn's team found that the quicksand--made from a basic recipe of sand, clay, and saltwater--is highly sensitive to the forces applied to it. Just a 1% change in stress caused the viscosity of the mixture to drop by a factor of nearly a million--a change that essentially turned the quicksand from solid earth to gooey liquid. This stress-induced liquefaction explains the admonition against struggling in quicksand, say the researchers. The more one pushes to get out, the more the quicksand turns to liquid.

If that was the whole story, then presumably one could gently swim out, but as every B movie watcher knows, quicksand grabs you and won't let go. To explain this, the team observed that quicksand with a sufficient amount of salt forms a rich sediment just minutes after liquefaction. Once it stiffens, "you're in trouble," Bonn warns. To pull a foot out at a rate of 1 centimeter per second would require the same force needed to lift a medium-sized car. Bonn advises those who find themselves in such a predicament to lift their foot very slowly and twist their leg to help water seep down.

Bonn's team also disproved the common misconception that victims sink to the bottom of a quicksand pit. Various objects placed in the mixture did not completely disappear, the researchers report 29 September in Nature. "Although you are trapped, you cannot drown," says Bonn.

The work nicely brings together granular mechanics and fluid chemistry, says physicist Heinrich Jaeger of the University of Chicago. But civil engineer Leslie Youd of Brigham Young University in Provo, Utah, disagrees with the idea that salt is an essential ingredient for quicksand. "I've stepped in freshwater quicksand before," he says. Bonn responds that quicksand without salt does not exhibit the same trapping sedimentation, and therefore is probably less dangerous.

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