Quicksand is a colloid hydrogel consisting of fine granular matter (such as sand or silt), clay, and salt water. In the name, as in that of quicksilver (mercury), "quick" does not mean "fast," but "living" (cf. the expression the quick and the dead ). Water circulation underground can focus in an area with the optimal mixture of fine sands and other materials such as clay. The water moves up and then down slowly in a convection-like manner throughout a column of sand, and the sand remains a generally solid mass. This lubricates the sand particles and renders them unable to support significant weight, since they move with little friction, behaving more like a liquid when exposed to stress. Since water does not usually go up to the surface of the sand, the sand on top appears solid, and can support leaves and other small debris, making quicksand difficult to distinguish from the surrounding environment.

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Properties

Quicksand is a non-Newtonian fluid: when undisturbed it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly. Someone stepping in it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of one centimeter per second would require the same amount of force as "that needed to lift a medium-sized car." ^[1]

It should be noted that because of the higher density of the quicksand, it would be impossible for a human or animal to completely sink in the quicksand, though natural hazards present around the quicksand would lead people to believe that quicksand is dangerous. In actuality the quicksand itself is harmless on its own, but because it greatly inhibits human locomotion, the quicksand would allow harsher elements like solar radiation, dehydration, or tides to harm a trapped person. ^[2]

Recent research findings

It was commonly believed that the behavior of quicksand was due solely to saturated or supersaturated suspensions of granules in water. Pressure from underground sources of water would separate and suspend the granular particles, reducing the friction between them. As of September 2005, it has been shown that it is the presence of salt that is largely responsible. The stability of the colloidal quicksand is compromised by the presence of salt, increasing the likelihood of sand flocculation and the formation of the high viscosity regions of sediment responsible for quicksand's "trapping" power.

Prevalence

Quicksand may be found inland (on riverbanks, near lakes, or in marshes), or near the coast.

One region notorious for its quicksands is Morecambe Bay, England. As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in rapidly.

See also

• Dry quicksand
• Fech fech
• Liquid limit
• Plastic limit
• Soil liquefaction
• Thixotropy

References

1. Khaldoun, A., E. Eiser, G. H. Wegdam, and Daniel Bonn. 2005. "Rheology: Liquefaction of quicksand under stress." ''Nature'' 437 (29 Sept.): 635. {{doi|10.1038/437635a}}
2. Discovery Channel. ''MythBusters''. Season 2. "Killer Quicksand." October 20, 2004.