Om's fine sand dune images took me back to a spring break during my undergrad days. I had heard about singing sands and managed to talk someone with a car into an expedition to the Mojave Desert. First towards Barstow and then onto a short dirt road before we got out and two hours of hiking to the Kelso Dune Field.
Some of the dunes are several hundred feet high and climbing is something you do when you're young or crazy. We were both and found a long leeward slope to slide down. I highly recommend sliding down dunes and this one was special. As we slid it made a satisfying rumble. I don't have a good sense of pitch, but would judge it somewhere between middle C and the C an octave higher. Over and over we slid just enjoying the effect. I poured some of it into a bag and we made it back to the car just around sunset.
There are perhaps two dozen examples of singing sands in the world. The effect hasn't been studied carefully enough to sort out exactly what's going on, but the common thread is you need very round grains of silica sand of similar size averaging about a quarter millimeter in diameter. One hypothesis is these clean, round equally sized grains act as layers that jiggle up and down as the shear force causes them to slide against each other. There are several other plausible ideas. Sadly it's been observed that even a little pollution can silence a dune. The Kelso Dunes are apparently a faint whisper of what they once were.
Singing sand is a different animal from the sand you tend to find on beaches and river beds. A few years ago it struck me that the sand found on great beach volleyball beaches doesn't stick to the athletes .. at least not that much. Sarah told me about - the official sand consultant for the Olympics and other major events Hutcheson Sand and Mixes in Huntsville, Ontario. Curious, I got in touch with their sandman and asked a few questions. You send them a kilo of your sand from layers down to about a half meter and they'll tell you if it meets the specification or find a source. The size and shape requirements of the grains (mostly between a half and one millimeter in diameter and naturally weathered to a sub-angular to rounded shape) insure it drains easily and doesn't compact like more angular sands. Size distribution and shape largely keep it from sticking to bare skin. The sand used in the London games came from a quarry near Surrey, Copacabana beach sand passed muster as do most Southern California beaches, but China had to import 17,000 tonnes from an island in the South China sea.
Much of the sand we normally see has a large distribution of size and tends to look more angular than round. The angular bits allow it to compact and, with a small bit of water, clump together and stick to skin. The binding force comes from the surface tension of small amounts of water than form little bridges between angular pieces. Not good for beach volleyball, but great for making concrete. A side note is demand has been so great that construction grade sand is getting rare and there are sand pirates. Desert sand doesn't cut muster, so all of those highrises in the Mideast require imported sand.
There are any number of interesting properties of sand, but one one you've probably seen. You may have noticed walking on sand can dry it out. Walking on wet sand applies pressure. The grains push against each other and, if they are angular, rotate into a new position. The sand had been fairly well packed by the force of wind and water, but many of these rotations force the grains apart a bit. This new configuration has a bit more empty volume and water the drains into it. It's called Reynold's dilatancy - here's a little video showing a simple experiment you can try at home.
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