Liquid Sand

One of the more fun physics stories that I've seen recently is from an area of research quite removed from my own, but that I have found fascinating for a while now. I have been fortunate to have excellent condensed matter colleagues at both my recent institutions, and quite a number of them are interested in soft condensed matter - classical physics that describes the behavior of large numbers of particles, far from equilibrium, often when entropic considerations dominate the dynamics. The field covers such diverse systems as the behavior of biological membranes and the dynamics of grain in silos, and contains many examples in which nontrivial geometry and topology lead to the possibility of discovering new phenomena that, unlike in my own field, can increasingly often be checked in a laboratory experiment designed and built in a relatively short time. The story that caught my eye (via Wired Science) recently concerns the behavior of a system that is so simple that you would think we know all that there is to be known about it - falling sand. In the video above, a stream of sand is allowed to fall over several feet, and is filmed using a high speed video camera that falls at the same speed as the sand. The result, as you can see, is that the sand forms "droplets" just as water would, even though most people would not think of granular materials as anything like a liquid. The work was performed by Heinrich Jaeger's group at the University of Chicago, and published in the current issue of Nature, which also deemed it worthy of a News and Views article and an Editor's Summary (subscription required for all these things, unfortunately). Interestingly, this system is still not fully understood - although it is clearly displaying liquid-like characteristics, the scales of the droplets and the forces involved are very different from the traditional regimes in which liquids are described - so there's still work to be done. You can see many other examples of behavior like this on Jaeger's granular materials page, with even more videos. The one I liked best is this granular jet one Remember - that ball isn't falling into a thick liquid - that's sand! There are apparently all kinds of applications of this kind of work. but I just think it's beautiful all on its own.