Why do some sand castles stand firm until swallowed by the rising tide, while others fall over instantly?

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.â€

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."