This is something that Aimee Whitcroft at the Science Media Centre in Wellington drew my attention to – thanks Aimee.
Most of us who have ever eaten breakfast cereal will probably be familiar with the phenomenon whereby the larger flakes of whatever-your-favourite-breakfast-is tend to be at the top of the packet, whereas the smaller flakes tend to accumulate at the bottom. This segregation of particle sizes is pretty common in physical processes. So it comes as no surprise that, when you stuff small beads of two different sizes into a container, and disturb them by rotating the container, the beads separate out in size, to give regions where large beads predominate, and regions where small beads predominate. That phenomenon has been known for seventy years – being first demonstrated by Yositsi Oyama.
But, what has been discovered recently by Ralf Stannarius and Frank Rietz is that, when you stuff even more beads into the container, and rotate it, the segregation patterns are not stable; instead you get rolling patterns reminiscent of convection currents (similar to the circulating currents in, say, a pot of water being heated from beneath). What is most intruiging is that there is as yet no full explanation of the phenomenon – the physical models that exist just don’t seem to be adequate for what is happening. You can look at the movie or, for those more physics-inclined, read the associated publication in Physical Review Letters.
That is what makes science so fun. Basically, with equipment you can put together in your own shed, you can demonstrate a phenomenon that is beyond our current understanding. We by no means know everything there is to know even about simple mechanical systems, as this example demonstrates. It begs the question as to what other effects are there that are just waiting to be discovered. So start experimenting.