Sandpile physics

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.

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