Sandpile physics

By Marcus Wilson 22/06/2011 2

Yesterday I had a look around the Tate Modern art gallery in London. As is the way with modern art, there was the expected mix of fabulous, bizarre and seriously-stretching-the-definition-of-art exhibits.

One of the pieces on show at present is a pile of porcelain ‘sunflower seeds’, by the artist Ai Weiwei. This piece of artwork is probably best known for being very politically-charged, the manner in which it is currently presented also illlustrates nicely a perplexing physics phenomenon: the ‘sunflower seeds’ form a large but nearly perfect cone.

If we pile up lots of small particles (e.g. grains of sand) we find that there is a maximum possible angle that the pile can take. If you try to make a slope that is too steep, you get an avalanche, until the slope reverts back to what is called the angle of repose. This angle depends on the particle size, shape and frictional properties. As we know, it is easier to build a sandcastle out of wet sand than dry sand, because the wet sand can hold a slope much better: it has a higher angle of repose.

Another example would be the conical nature of many volcanoes, e.g. Ngauruhoe. If you model this (perhaps simplistically) as a source of cinder at the top of the volcano then a cone shape naturally follows, since the slope everywhere on the cone is the same.

Although the effect is very clear, the exact reasons as to why this happens is not; it’s still a subject of a lot of research.

2 Responses to “Sandpile physics”

  • Many years ago I worked at the local Highway Dept. for my summer job. When I would get on my boss’s bad side I would be sent to the sand pit to screen sand or loam I had a front-end loader, a screening machine and lots of time to think about what I was doing. I became an expert on two things: the rate of change of the height of the pile (because I would have to move the screening machine), and also the angle the piles would make as they grew into huge inverted cones.

    When I finally got the nerve to tell my boss the discovery I had made (The sine of the angle is proportional to the average grain size. I did not know if it was true, but it was useful for me) my boss said “Of course it is, it’s called the angle of repose! You can look it up in the little engineer’s handbook!”

    Boy, did I feel dumb. I figured then that any revelations I had about dirt probably weren’t too original.

    • That’s a great story to tell. But it shouldn’t have made you feel dumb – rather it should have made you feel that you were a proper scientist, having worked this out by your own experimental observation. I’d hazard a guess that if your boss had told you about the angle of repose on your first day of work you’d have dismissed the information as completely irrelevant…

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