ScienceDaily (Aug. 2, 2012) — All children who build sandcastles on the beach know that in addition to sand you also need to add a little water to prevent the structure from collapsing. But why is this? In an article which appeared today in Scientific Reports from the publishers of Nature, researchers from the University of Amsterdam’s (UvA) Institute of Physics (IoP) answer this question.
The function of water in sandcastles is to form small 'bridges' which make the grains of sand stick together, thus increasing the solidity of the structure. The researchers show that the optimum amount of water is very small (only a few per cent). If this optimum concentration is used, sandcastles reaching five metres in height can be built.
While it might sound flippant, building the best structures out of sand is a tricky civil engineering problem. That's why a team of scientists, led by Daniel Bonn from the Laboratory of Physical Statistics in Paris, set to a complex series of experiments involving theoretical and practical modelling of sand castles.
By testing different levels of sand wetness, they studied how columns of sand buckled as they were built higher and higher. Eventually, they found that there's an optimum sand wetness: you should be aiming to have a liquid volume fraction of 1 percent. That means that, by volume, you need to combine 99 parts of perfectly dry sand with 1 part water to build the ultimate castle. The result is published by Nature.
How to construct the perfect sandcastle
Maryam Pakpour, Mehdi Habibi, Peder Møller & Daniel Bonn
Scientific Reports 2, Article number: 549 doi:10.1038/srep00549
Received 11 June 2012 Accepted 17 July 2012 Published 02 August 2012
Just a bit of water enables one to turn a pile of dry sand into a spectacular sandcastle. Too much water however will destabilize the material, as is seen in landslides. Here we investigated the stability of wet sand columns to account for the maximum height of sandcastles. We find that the columns become unstable to elastic buckling under their own weight. This allows to account for the maximum height of the sand column; it is found to increase as the 2/3 power of the base radius of the column. Measuring the elastic modulus of the wet sand, we find that the optimum strength is achieved at a very low liquid volume fraction of about 1%. Knowing the modulus we can quantitatively account for the measured sandcastle heights.
Figure 1: Sandcastles with diameters 2 cm and 7 cm.
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 meters. If Dr Bonn is right, sand engineers could in principle beat that with a castle thrice the height upon the same foundation.