Sand Dunes

February 14, 2012

Dune was a 1965 science fiction novel by Frank Herbert. Although the book and its sequels were quite popular, the 1984 film based on the book was uniformly derided.[1] I'm a science fiction fan. Although given multiple opportunities, I've never been able to watch this film in its entirety. It is entertaining, however, to see Sting, who played Feyd-Rautha, as an actor.[2]

The setting of Dune, as its name indicates, is a desert planet filled with sand. Sand is a granular material, and there is much physics in both a grain of sand and a handful of sand. One of the more interesting phenomena involving sand is singing sand, a very loud noise made when desert sand dunes slump or avalanche from wind action or human trespass.

Beach sand, Ricadi, Calabria, Italy My father's family is from Calabria, although the Italian region known as Gualtieri is far to the north. (Photo by Manfred Morgner, via Wikimedia Commons).

Desert singing sands produce low frequency sound, but some dry beach sands make high frequency sound when they are walked upon. There is no precise understanding of the mechanism for this, although it's been found to occur mostly for highly spherical grains. The sound may arise from a stick-slip friction between rubbing grains much like a violin bow rubbing against a string. The grains would move in layers atop other layers to produce the sound.[3]

I wrote about another interesting granular phenomenon in a previous article (Granular Materials, June 12, 2007). That's the well-known Brazil nut effect, named after the tendency for large nuts to be at the top of a container of mixed nuts when it's opened. This phenomenon is not restricted to nuts. It occurs in any dry, granular mixture of large and small particles when it's shaken vertically.

The mechanics of granular materials are quite strange. The highest pressure at the base of a conical sand pile is not at its center, but in a ring at one third the radius of the base. The flow rate of sand through an hourglass is independent of the height of the sand in the upper chamber, quite unlike the case for water.

Another unusual effect arises from the combination of friction and triboelectricity, as I described in a previous article (Triboelectricity, July 17, 2007). If two different types of sand grains (for example, two colors of "art sand" found in craft stores) are mixed in a shaker and then poured into a beaker, the particles will segregate.

The shaking action gives the particles a positive charge as electrons are stripped away by friction, but one type will lose electrons more easily than the other and become more positively charged. The repulsive forces of the electrical charges will "unmix" the mixture. Sounds like a spectacular, but simple, science fair experiment.[4-5]

All the preceding experiments were done with silica grains, but there are regions on Earth where sandy terrain is composed of gypsum, a mineral form of calcium sulfate dihydrate (CaSO₄·2H₂O). Most homes, including my own, have large quantities of gypsum built into them in the form of drywall (a.k.a., sheetrock, or gypsum board). Drywall is an interior wall board composed of a thick layer of calcined calcium sulfate (CaSO₄·½H₂O), reinforced with fiberglass and enhanced with various additives, between paper sheets.

One of these gypsum sand regions extends around the White Sands National Monument in New Mexico. These dunes have been the recent object of study for Douglas J. Jerolmack, Assistant Professor in the Department of Earth and Environmental Science, the University of Pennsylvania, his students, and his collaborators from the University of Alabama, the University of Pennsylvania and Temple University.[6-7]

Gypsum dunes at White Sands, New Mexico. (National Park Service photograph, via Wikimedia Commons).

White Sands National Monument is the 275 square mile dry bed of a lake that existed during the last ice age.[6] Although the surface is arid, there's a water table just a meter below the surface. The presence of this moisture makes the gypsum sand somewhat sticky, so it behaves differently than desert sand. This is one of the factors that complicates the evolution of the surface geology.[6]

As for other sandy places, wind is the predominant factor in sculpting the surface topography. There are places where the wind pushes the gypsum around quite a bit, but there are other places where the motion is so slow that vegetation has grown. The presence of the vegetation further slows the movement of gypsum sand. What's specifically interesting is that, because the vegetation inhibits dune movement, which subsequently allows more growth of vegetation, there's a rapid transition from bare dunes to dunes almost entirely covered with vegetation.[6]

This mechanism gives a warning of one possible consequence of global warming. Even a small reduction in rainwater in places such as Sand Hills, Nebraska, might extinguish the grasses that stabilize its dunes, causing the region to revert to a barren landscape. This would have disastrous consequences for the half-a-million cattle that now graze there.[6] Says Jerolmack, "It happened during the Dust Bowl, and it could happen again.[6]

The study was supported by the University of Pennsylvania, the National Science Foundation and the National Park Service.[6]

References:

1. Dune, 1984, David Lynch, Director.
2. Sting as Feyd Rautha in Dune (YouTube video).
3. Vincent Gibiat, Eric Plaza and Pierre De Guibert, "Acoustic emission before avalanches in granular media," arXiv Preprint, June 20, 2009.
4. Phil Schewe and Ben Stein, "Spontaneous Separation of Charged Grains," Physics News Update, Number 832 (July 12, 2007).
5. Troy Shinbrot, "Tribocharging of Identical Materials" Rutgers University, Biomedical Engineering.
6. Katherine Unger Baillie, "Penn Researchers Uncover a Mechanism to Explain Dune Field Patterns," University of Pennsylvania Press Release, February 6, 2012.
7. Douglas J. Jerolmack, Ryan C. Ewing, Federico Falcini, Raleigh L. Martin, Claire Masteller, Colin Phillips, Meredith D. Reitz and Ilya Buynevich, "Internal boundary layer model for the evolution of desert dune fields," Nature Geoscience, Document No. 1381, February 5, 2012.