Sand, Water, and Science: the perfect sandcastle recipe

Making sandcastles is a core part of what makes going to the beach so fun! Both adults and children join in, equipped with plastic buckets and spades, toiling away under the sun. But how is it possible to make sandcastles? How is sand mouldable when wet, but fine and grainy when dry? 

The secret of every sandcastle lies in the physics by which water molecules hold together the millions of sand grains in your bucket. This might be a chemistry blog, but you can't understand chemistry without understanding how biology and physics interact in the natural world.

It all comes down to two words: surface tension. Surface tension occurs due to the hydrogen bonds between adjacent water molecules causing molecules to 'stick' together. This can support weight, allowing small insects to walk on pond water and needles to float on top of water though it is much more dense.

In a sandcastle, the water molecules coat the grains of sand and hold them together with their surface tension. If you add too little water, there won't be enough to coat the grains fully and not enough surface tension to stick the sand together. Add too much water, and the sand will behave is a more fluid manner as the grains aren't held in place.

So what is the ideal recipe to make the perfect sandcastle? Researchers suggest an 8:1 water to sand ratio will ensure all the sand grains are well coated to optimise the surface tension.

However, that's not all! The type of sand you use also affects how strong your sandcastle is. Angular grains e.g. beach sand, lock together better to create stronger structures compared to rounded grains e.g. desert sand. A mix of grain sizes improves stability as well as the smaller grains can pack into the gaps between the large grains. 

Friction between sand grains when you are compacting sand can also add stability and helps the structure resist collapse. Shaping your sandcastle to spread out the weight more evenly e.g. in a pyramid shape, can reduce pressure and increase stability. And, of course, the more stable your sandcastle mixture, the taller the sandcastles you can build!

Next time I go to the beach, I know I'll be using some of these techniques!