The Sand Reckoner

Driving home one night last week, Auckland’s Spaghetti Junction was more than normally congested and my thoughts turned to physics. Sluggish traffic provided my initial topic – the differences between sand and water. – as well as letting me follow my train of thought (an actual train would be a welcome option, but that’s another story) without endangering my fellow road-users. Admittedly, many of the differences between sand and water are obvious to anyone who visits a beach, but I was thinking about how sand sometimes flows like a liquid.

Anyone who has seen an old-fashioned egg timer knows sand can flow, but flowing sand is not the same as flowing water. If a hose is leaking from a pinprick, increasing the pressure makes the water flow faster. On the other hand, pressing down on sand flowing through a narrow funnel can cause the grains to lock together, stopping the flow entirely. The technical term for this is “jamming”, which is how I got to be thinking about sand while sitting in traffic.

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Simulation showing spontaneous jamming in a granular material.

Physicists often explain the properties of “granular materials” like sand by looking at interactions between adjacent grains. The same reasoning can be used with other systems – the complex movements of flocking birds are reproduced by boids, “birdoid-objects” that obey a few simple rules; avoid collisions but stay close to the flock…

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Simulated starlings…