Monique Keiran: The science of sandcastles, and why it matters

With the early-summer heat wave sending many people ­scurrying to local beaches, Nature Boy is using his seaside sojourns to ponder the science of sandcastles.

Yes, such a thing exists, and it’s serious stuff. Nature Boy, physicists, geoscientists and other pointy-headed people devote considerable grey matter to the contemplation of single and billions of grains of sand ­artfully sculpted.

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I mean, really, who wouldn’t?

I particularly like the account of the British geoscientist paid by a travel company to ­determine the secret to ­building majestic sandcastles that last. Naturally, the crucially ­important study required that the sand of the U.K.’s most ­popular beaches be sampled and compared for sandcastle-­building qualities. Equally important was a comparison of each beach’s esthetic qualities.

The verdict may come as a relief. Esthetics are a matter of personal preference, and any sand will do, although some is better than others.

Dig beneath the puff, ­however, and you find complex science. What enables wet sand to clump together and to be shaped into castles encompasses some of the most basic physical phenomena that allow our world to work the way it does.

At play are two small but mighty forces of nature. The adhesive force is the clinginess between two different kinds of particles — between water and the surface of a grain of sand, for example. The cohesive force is the clinginess between ­particles of a similar type — between water molecules, for example.

These different kinds of clinginess contribute to two other phenomena at work in ­sandcastles. Because molecules in a liquid cling to each other (cohesion), this causes the ­molecules on the liquid’s ­surface to contract and resist being stretched or broken.

Voilà! We have surface ­tension.

When the clinginess between a liquid and a ­surface ­out-­muscles the clinginess between the liquid’s own ­molecules, ­capillary action results. ­Capillary action allows liquid to be drawn into and flow through tiny spaces — such as into the nib of a pen, up between the fibres in a candle’s wick, or between tightly packed grains of sand.

Capillary action is essential to life on Earth. It overcomes ­gravity to bring water and nutrients from roots deep ­underground up hundreds of feet to the leaves in a tree’s canopy.

It draws blood through our vascular system’s ­tiniest ­channels — ahem, our ­capillaries — and enables the transfer of nutrients and blood plasma in and out of our cells. It allows nutrients to pass through our intestinal walls and makes ­possible the exchange of oxygen and carbon dioxide in our lungs.

In terms of the sandcastle you’re building on Willows Beach, when water is added to dry sand, capillary action draws the water into the tiny spaces between the sand grains. Once there, the water molecules form tiny flexible water-drop bridges (cohesion!) between ­neighbouring sand grains. These bridges are held together by our old friend, surface tension.

As more water is added, the small bridges between grains grow into bigger bridges. As the bridges grow, more of each grain’s surface comes into ­contact with more water, which increases the water’s binding effect.

As yet more water is added, the bridges between sand grains combine and act to bind several grains together into ever-larger structures. Now we’re ­starting to get the sand clumps that are so necessary to successful ­sandcastle construction.

When every grain of sand is coated with water and the sand-to-water ratio is just right — not too low, not too high — the water-molecule bridges are the perfect strength for piling and sculpting sand into sandcastles. The optimal sand-water ratio has long been believed to be about eight parts of sand to one part of water, but some physicists ­determined that just one per cent water is best for building stable, five-metre-tall sandcastles.

And then, as anyone who has watched an advancing tide swamp a sand fortress knows, too much water overwhelms the whole system, causing the clump to disintegrate into sand soup.

The applications of ­sandcastle science extend well beyond the beach. They can apply to any granular substance — grain in silos, sawdust piles in mills, soil in pits or even plastic balls in McDonald’s play rooms, for example. They also apply to how the ground behaves during earthquakes and in the lead up to landslides.

All that aside, Nature Boy is now wondering how he might obtain funding to study the sand qualities at Vancouver Island’s best beaches.

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