Sand, Man

by Clifford Johnson     Department: Physics & Chemistry

In physics, most of what we do is look for the simple, often in extremely complicated systems. If you're asking the wrong question, or looking at the wrong aspect of the system, this quest for the simple is unlikely to work at all, but the right question asked about the right aspect can yield rather striking insights, often with far-reaching consequences. Although it often is not emphasized in this manner during our school or undergraduate (and sometimes even graduate) education, this is the primary skill in the physicist's arsenal that we teach and learn. (See an earlier article here for a take on this.)

Sometimes, you don't need the sharp eyes and years of training and drilling in these seemingly arcane (but, I'd argue, most natural) arts (and the requisite sequestering away in monasteries and nunneries with abstinence, self-flagellation, and so forth) - there are times when if just jumps out at you that there's a simple question or two begging to be answered, one simple mechanism at work. A fine example presents itself to me whenever I visit the desert sand dunes in Death Valley, as I did last week. (The title of this post was chosen to remind me that I suddenly realized while in the act how ironic it was that one of the things I was reading by the campfire near the dunes was Neil Gaiman's "The Sandman" .) On the one hand, the sand is hugely complicated, with myriad sand grains, like, well, grains of sand, each with every intention to go wherever they like, not being bound to any other grain of sand. Somehow, however, the first thing that one sees - physicist or not - is that there's a huge amount of order all around. Almost an unreasonable amount, in fact.

There are ripple patterns of a striking degree of regularity on small scales underfoot (as shown to the right). There are little crescent shaped small dunes allied to these too, as shown below left. A glance up at the larger dunes - some as big as buildings - is rewarded with similar structures leaping out at you, this time writ large. They're intersecting with each other in places, but unmistakably they are the same shapes. What do I mean by "the same" here? They're clearly not identical, are they? Well, yes and no. One of the most striking persistences is the geometry involved, when there's a single wind direction. The shaping force here is the prevailing wind (or breeze, for it need only be gentle) coming from a specific direction over time.

The sand marches forward under the influence of the wind, forming these dune shapes, their backs making a gentle upward slope that is quite gentle (about 15^o), and more or less the same on all dunes, big as a house, or small as a mouse. Their fronts (the leeward side) are quite different however, making a much steeper angle (about 33^o) which again is the same on every dune. Remarkably so. It is governed by a simple slipping phenomenon that is independent of the wind (since that side is more or less shielded from wind by the rest of the dune. These leeward slopes are really fun to run or roll down too. Just the perfect steepness - any steeper would be alarming, and any shallower, dull. I don't know if that's an accident. A given area of sand will have those two angles appearing all over it for as far as the eye can see. It is quite striking to see. (Note that in my primary photo at the top of this post I have ripples forming on top of a huge dune: a combination of two features)

There's a relatively small variety of basic structures that appear, given the apparent freedom the system has to do what it likes, and understanding them is an important field that touches on several areas of science, from theoretical physics to geology and matters of environmental and habitat preservation. This field is quite old, as you might imagine, and the first modern book on the subject is the lovely one by Bagnold in 1941 (Bagnold, R. A. 1941. The Physics of Blown Sand and Desert Dunes London, Chapman and Hall.)
which apparently remains a heavily used reference today. It reports on lots of descriptions and experiments by the author, and is (I've heard) quite a lovely book. I must look out for it. The classic dune shape I've been describing (both it and the ripples are formed by what geologists calls a "saltation" process driven by the wind, involving individual sand particle bouncing along on top of each other) is the "Barchan" dune. There's a lot more online, such as this article, or this one, or even this amusing and pleasant digression.

There's been a huge amount of research in the subject since that book of course - it has not stood any more still that the wind-coaxed sand dunes themselves - and I encourage you to explore it yourself (there's any number of articles online), as there are several remarkable avenues of exploration, and connections to other fields (one of my favourites being to critical phenomena).

Yes, it's definitely a wonderful and beautiful world out there, if looked at the right way. Physics can be one of those ways.

-cvj

(Cross posted to Asymptotia.)

Tags: death valley, patterns, sand dunes

TrackBack URL for this entry: http://www.pbs.org/mt4/mt-tb.cgi/549