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The ‘science’ of sand castles

The summer holidays are nearly upon Australians. When I was a kid, my family would spend summers at the Jersey shore. One pastime was building sand castles. I use that term generically to include sand castles, sand sculptures and – in my case – sand blobs!

My handiworks were nothing like the creations of serious sand castle devotees on the beaches (and elsewhere) today. My wife Pam and I saw a dragon sculpture (pictured) on a visit to Australia. Now that’s a sand sculpture! The sucker even had fire coming out of its nose!

{{quote-A:R-W:300-Q:"The man who stands firm in order to protect a sand castle can never be relied upon – for he has given away his common sense."}}This kind of stuff is not kids’ play. One problem is sand, by itself, is next to useless as a building material. Without some kind of binder, it simply slumps into gentle mounds. Most often, in the case of sand castles, the binder of choice is plain old water.

You need just the right amount of water. If the sand is too dry it will not stick together, no matter how hard you pound on it. But don’t get it too wet either, or it will ooze all over the place.

Scientists who have studied the subject generally agree that the optimum water content is about one to two per cent by volume. However, the sand sculptors I have talked to generally determine the right consistency by what feels right. In either case, tell your kids they can’t just grab a bucket full of damp sand. This is serious business!

You should also tell your kids that when using the optimal water to sand ratio, their castle can reach a maximum height roughly equal to the radius of the base to the power of two-thirds – or Hmax ~ R2/3.

“What?” you say. “Are you kidding?” OK, just tell your kids that the higher the castle, the wider the base, and be sure to thoroughly tamp the sand.

If this doesn’t ruin your kids’ sand castle experience, you can tell them why damp sand behaves as it does. Most of the pore space between sand grains is filled with air; there is just a small amount of water in the pores.

{{image2-a:r-w:200}}Surface tension – the same force that causes water to climb up the inside of a glass – causes water to form little grain to grain bridges. These capillary bridges act like rubber bands holding the grains together. Adding more water to damp sand fills the spaces between the grains and the bridges disappear, causing the sand to slump.

Those capillary bridges are what cause sand to stick to your skin when you lie down on the sand after a dip in the ocean.

When you dry off, the sand grains easily wipe off your body because the water, and thus the capillary bridges, is gone.

Show your kids how this works by putting a little water on your index finger and thumb. When you move your finger and thumb towards each other, the water coalesces to form a small water bridge between your fingers. That’s a capillary bridge.

So next time your kids build a sand castle, make them follow these procedures. But if you think I might have gone overboard on the subject, you are not alone. To quote the great Winston Churchill: “The man who stands firm in order to protect a sand castle can never be relied upon – for he has given away his common sense.”

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