A recipe for the construction of the perfect sandcastle is revealed in a paper published in Scientific Reports this week. The study suggests that the most stable sandcastles have a very low water content of about 1%, but the maximum height a sandcastle can reach also depends on the diameter of its base. The work could have applications for civil engineering and soil mechanics.

The stiffness of sculptured wet sand in a sandcastle results from the formation of capillary bridges between the sand grains - dry sand, conversely, can hardly support its own weight. Sand that contains only a small amount of water can be sculpted into an impressive sandcastle, but too much water will destabilize the structure, as seen in landslides. Daniel Bonn and colleagues investigated the stability of cylindrical columns of wet sand, both theoretically and experimentally. The columns become unstable when they undergo a buckling transition under their own weight. The authors find that the maximum height of a sand column increases as a 2/3 power of the radius of its base. By measuring the ratio of the force exerted on the sand to the resultant deformation, or the elastic modulus, they also show that the optimum strength of wet sand is achieved with a very low liquid content. This model allows the authors to calculate a sandcastle’s maximum height.

One way to build a more stable and therefore a taller sandcastle is to decrease the effective density of the sand, which can be achieved by plunging the structure under water. This will destroy the liquid bridges between the grains of normal sand, but if hydrophobic sand is used instead one can have air bridges between the sand grains under water.