The Science of Sand Sculpture

Have you ever gone to the beach and made castles or other sculptures in the sand?

If you have, you know that wetter is better. Dry sand is something you dig through to get to the good stuff.

But if a friend asked you why wet sand is better, or how much wetter the sand should be to make all those turrets, tunnels, and archways, would you shrug and say, "I dunno"?

For Peter Schiller, the wet-sand mystery was one that begged to be solved. One day, he says, "I was trying to put up a swing set for the kids, and it had to be weighted with sand" to keep it from tipping over. The sand he was using was damp, and as he tried to pour it, it came out in clumps rather than in a steady stream. He wondered why.

Question: How much water?

Dr. Schiller studies and teaches physics at the University of Notre Dame in South Bend, Ind. He worked with a fellow scientist and some of his students to figure out what was going on. Scientists have been working in, around, and with sand for thousands of years. But they are still puzzled by some of the simplest things sand does. Why does it clump together, or hold a shape? Why does dry sand form the shape it does as it falls through an hour glass?

Researchers are pretty certain about the forces at work. When sand is dry, friction between sand grains and the shape of each grain determine the shape that sand makes as it piles up in the bottom of an hour glass. The rougher the grains, the steeper the sides of the pile as the grains "grip" each other.

When sand is wet, the water acts like glue. It holds the grains together. You may have noticed the "water glue" principle at work when you helped rinse the dinner dishes last night. Two wet plates sometimes seem as if they're stuck together. A thin layer of water forms a "bridge" between the two plates. The water's surface tension keeps each plate stuck to its side of the water bridge.

Testing sugar, rock, coal

But seeing is one thing; measuring is another.

Researchers have had a tough time measuring the effects of water on grains of material. They've tried, using everything from sugar and seeds to rock chips and coal. But the results haven't been conclusive.

Instead of sand, Schiller used tiny plastic balls about 1 millimeter across. Instead of water, which evaporates quickly, he used corn oil and lubricating oil.

He and his team "dribbled" the plastic sand through a hole in the bottom of a bucket. As the balls fell through the hole, they left a crater in the bucket. (You see similar "craters" in the top part of an hour glass.) The crater was so narrow, he couldn't measure its slope (how steep it was) directly. But by weighing the balls that fell through, Schiller and his team could figure out how the coating was affecting the way the particles stuck together.

Answer: a tiny amount

Schiller found that it didn't take much moisture to get grains to clump together. The layers of oil coating the grains were never more than a few molecules thick. The more moist the grains, the higher they would stack and the steeper the slope on the sides of the crater. When they looked at how much of that small amount of oil actually was doing the gluing, they found it was a very small amount indeed: only about 0.1 percent of the layer.

You may have conducted your own seaside version of Schiller's experiment (minus the fancy measurements) without knowing it. Have you ever made sand castles by dribbling very wet sand through your fist? You can build very steep towers using very wet sand.

And if you tire of dribble castles, you can try to "read" the sand's history. If you're on South Padre Island, Texas, the sand you dig probably started out as boulders on the sides of the Rocky Mountains some 20 million years ago. In the Pacific Northwest, the black-, green-, or gray-sand beaches tell of the sand's origins as volcanic rock. And in New England, glaciers ground granite into sand.

Not bad for a day at the beach!