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Treebeard's Stumper Answer
14 April 2000

Wet Sand, Dry Sand

We had a fine time at El Capitan beach last week after our long hike and overnight campout. We didn't last long in the cold ocean water, but the sand was warm. Sand and water are interesting materials that we take for granted. Damp sand that's not too wet holds together just right for sand castles or a game of alligator, our Hike Club beach version of Jenga. You can judge how damp the sand is by its color since wet sand is a darker color than dry sand. Water is not sticky or colored, so how can it hold sand together and change its appearance?

This shows the color difference between wet sand and
dry sand at El Capitan beach. The tide is rising up over
the dry sand. Notice how successive high waves leave
layers of color and detritus. That's another stumper!
Alligator is an old surfer game. Make a pile of damp sand
and push a stick in the top. Take turns removing as much
sand as you dare. Whoever brings the stick down becomes
the alligator and has to crawl into the water while everyone
else chucks sand at them. This is the original Jenga game!


The surface tension of water holds sand castles together like a pond supports water striders. Thin films of water are strongest, so fine clay is stickier than coarse sand. Try separating two pieces of wet glass to appreciate the effect. Wet sand, clothes, and hair all appear darker because of refraction. Just as a straw looks bent in a glass of water, so sunlight is bent inward by the thin layer of water between sand grains. Less light is reflected to our eyes, so the sand looks darker. There will be more interesting science at the DMS Science Fair on May 12!

Notes:

Actually water is sticky, though it sounds strange to say it. Water sticks to our skin after swimming, and it sticks even more to the towel when we dry off. Water also sticks to itself, which is why drops stand up with a rounded shape rather than flattening out into tiny puddles. That's surface tension.

This all has to do with the molecular structure of water, though in general it seems backwards to me to explain the visible in terms of the invisible. Water is H2O, an oxygen and two hydrogens bound in a molecule. The outside hydrogens attract each other to an angle of 105° across the oxygen like tiny Mickey Mouse ears. The result is that the dipolar water molecules have a plus and minus side, which can attract and repel like tiny magnets. Opposites attract. In liquid water, the charges are balanced in all directions, but the surface molecules line up in a skin-like layer held together by hydrogen bonds. Many of the remarkable properties of water as a heatsink and solvent result from this assymetrical molecular geometry. It also holds sand castles together. It's relatively easy to study in the lab. This is not a strong force, but sand has a lot of surface area so it adds up, like the tiny hooks and loops of velcro.

Water striders exploit surface tension by having waxy feet that are not wettable. They actually push into the surface film and make indentations that produce the cool shadows. With patience you can float a needle or even a paper clip or a piece of hardware cloth. If you magnetize the needle first by stroking it with a magnet, you can make a compass! A bit of detergent reduces surface tension enough that even water striders will sink. Are they less common in polluted waters?

Graybear adds this comment:

If you drive your car on the beach you should stay a certain distance from the water. Too far away and you'll get bogged down in the loose, dry sand. Too close, and the waterlogged sand will flow like mud. The right amount of water in the sand, through capillary action and surface tension, will provide a firm surface on which to drive.
The same goes for walking and jogging on the beach. There's a narrow zone that's "just right." I've also noticed that a parked car can sink into the same wet sand that you can drive over just fine. And firm sand will liquify if you "patty-cake" it with your hands or feet.

Sand is tricky stuff!

Here are a few Web links for further research. There are many more!

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Copyright © 2000 by Marc Kummel / mkummel@rain.org