Castles made of sand

It's one of the key lessons for beach-bound toddlers: if you're going to make sand castles, you've got to plop your tushy down near the water. That's where you'll find damp sand -- dry sand just won't hack it.

But why does damp sand magically hold the shape of a yogurt container after you dump it out?

Amazingly enough, only recently have scientists approached this earth-shaking question as a problem of basic physics. Still, the influence of dampness on tiny particles is a big deal for any factory that handles powder, in industries ranging from pharmaceuticals to agriculture. (Plugged plumbing is not just a problem in the cardiac biz.)

In research reported on June 18, 1997, in the science journal Nature, Notre Dame University physicists Peter Schiffer, Albert-Laszlo Barabasi and colleagues have pinned the clumping of damp grains to the same phenomenon that causes water to bead up on a waxy Ferrari.

On a Ferrari, it's called "surface tension." In a sandbox, damp grains stick together by what Schiffer calls "interstitial liquid bridges."

To measure the phenomenon, Schiffer and his group rounded up some polystyrene spheres, each 0.8 millimeter in diameter, and mixed them with a smidgen -- less than 1 percent by weight -- of oil.

Then they put this mess in a container, and measured the angle of the cone that formed when they pulled the plug on the bottom. As they gradually increased the oil content, the glop began holding a steeper angle. Then, rather suddenly, "Clumping takes over, and instead of particles moving as individuals, they move as a clump," Schiffer says.

Photo by Dave Tenenbaum.

At this point, the sand-castle builder could start crafting parapets, crenellations, and other architectural details.

Because they were using plastic spheres instead of irregular sand grains, the Notre Dame researchers could calculate the tiny amount of fluid -- less than one-ten-millionth of a cubic millimeter -- needed to hold each ball together. Almost all of the oil was doing something else -- like coating the spheres, or hiding in crevices.

We haven't gotten to the essential question: what held the balls together? Apparently tiny bridges of oil that formed between them.

Why oh why?
OK, smarty pants. So why did the tiny bridges form?

It turns out, Schiffer explains, that molecules of fluids (gases or liquids) "prefer" to be in contact with molecules that give them the lowest potential energy. In some cases (think waxy Ferrari) a water molecule is most content when the water beads up. In other cases (think beach towel) it's happier to be spread out in contact with the solid material.

And when you pull apart two grains connected by a liquid bridge, Schiffer adds, "the system is not as happy, energetically speaking, as when you keep them connected." To a physicist, that equals clumpiness. To a castle-builder, that equals sculpture.

We admit that we're talking thermodynamics here, but we promise to stop after explaining that when water in a sand castle bridges the gaps between the grains, it adopts a shape that minimizes its energy. In the process, it acts as a kind of glue, and just a little dab will do.

So how can this help us on the beach? Maybe it can't, says Schiffer, who denies being a builder himself. "I'm not sure there's a take-home message for building sand castles."

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