How do you make the perfect sandcastle? A little water can give you a five meter high castle

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.

The research team led by Daniel Bonn, Professor of Complex Fluids at the UvA, tested the theory successfully on cylindrical 'laboratory' sandcastles. They also show that with specially treated water-repellent sand, even an underwater sandcastle can be built (see photo). The water that serves as glue when building normal sandcastles is substituted with air bubbles when building the underwater versions.

The results are of practical importance to the civil engineering and soil mechanics sectors, the fields of science that deal with the stability of soil structures.

The research was conducted by the IoP-UvA’s Soft Matter research group, who are examining the special properties of soft materials such as polymers, emulsions, or granular materials (sand).