A day out on the beach would be incomplete without a sand castle. The mightier the castle, the better. But sand is next to useless as a building material. Without water it simply spreads out as wide as possible. So in search of a good recipe Daniel Bonn, a physicist at the University of Amsterdam, and colleagues have stumbled upon a formula for making the perfect sandy redoubt.
As they reveal in a paper published this week in Scientific Reports the key is to use sand with only 1% water by volume. Wet sand has grains coated with a thin layer of water. Owing to water’s surface tension this thin coat acts like skin stretched over many grains, holding them together by creating bridges between the grains. The strength of these bridges is enough to fight Earth’s gravity and prevent the structures from buckling under their own weight.
An easy way to achieve the right amount of water, Dr Bonn suggests, is to tamp wet sand in a mould (open at the top and the bottom) with a thumper at least 70 times, as he did in his experiments.
As for the design itself, unsurprisingly, the wider the base the taller the castle. According to calculations, using ideally moist sand, a column with a three inch diameter could rise as high as two metres. At 12 metres, the current world record for the tallest sandcastle, set by Ed Jarrett in 2011, used a base of roughly 11 metres. If Dr Bonn is right, sand engineers could in principle beat that with a castle thrice the height upon the same foundation.
First published in The Economist.
Image from here.