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Why do some sand castles stand firm until swallowed by the rising tide, while others fall over instantly?
By Elizabeth Svoboda
|
January 2006
Sand castles are ephemeral by natur—a good swift kick will destroy the
sturdiest silicon-dioxide fortress in seconds—but scientists have
learned that there are ways to maximize your creation's strength. Clark
University physicist Arshad Kudrolli recently published a paper in the
journal Nature Physics on just this topic, entitled"Maximum Angle of
Stability of a Wet Granular Pile.”
A well-engineered sand edifice,
Kudrolli says, must possess the ideal sand-to-water ratio. He and his
colleagues, who had long studied the properties of granular materials,
set out to find this ratio by mixing glass beads with various amounts
of water. They placed a pile of each sample in a rotating Plexiglas
drum and used a digital camera to record how far they were able to tilt
the drum before the pile collapsed. The more extreme the tilt a mixture
could survive, the higher the team rated it for structural integrity.
As any sand sculptor worth his or
her plastic shovel might guess, the strongest piles turned out to be
the ones made from relatively damp sand. The reason: Water forms
so-called liquid bridges in the narrow channels between sand granules.
The high surface tension of these tiny bridges makes individual grains
adhere into durable clumps. In other words, Kudrolli explains,"The sand
grains come out sticky. They form a heap that's more stable.” The
effect has its limits, though. If too much water is added, the sand
particles are"drowned” and no liquid bridges form. So what's the ideal
recipe?"About 8 parts sand to 1 part water," Kudrolli says."Although
our data indicate that anything from 6:1 to 20:1 sand-to-water would
work."
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