A Man's Vacation Home is His Sandcastle

How to construct the perfect sandcastle

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.

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  1. We then posit that detergency lowers surface tension and ruins the binding. Supressing vapor pressure (surface evaporation) and “salting in” stabilizes enhances binding. Now, the fun part – reductions to practice.

    Chaotropes guanidinium, tetramethylammonium, or tetrabutylammonium thiocyanate, perchlorate, or hexafluorophosphate increase the surface tension of water. Sand would bind more strongly. Kosmotrope aluminum sulfate would put sand castles in the pickle. Aluminum citrate would be interesting for its internal chelation rather than direct water hydration.

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