26 August 2007
In the above video, grains of dry rice are placed on a flat sheet that is driven at different frequencies (the text implies that the surface is a speaker, but it could also be done with a metal sheet or somesuch). At certain frequencies (depending on the size, shape, and material) there are spots on the sheet that tend to stand still (nodes) and spots that wave up and down a lot more (anti-nodes) - these key frequencies are called resonances. If a grain of rice lands on a spot that waves up and down a lot, it will be knocked around, but if it lands on a stationary spot it will act just like it was on a normal table surface and stand still. Thus the grains of rice tend to gather around the nodes in the material when at a resonance frequency - as shown by the pretty patterns.
As the frequency changes away from a resonance, all of the material will shake up and down (no nodes exist), and the rice will scatter all over (these are also the times when the person adds more rice). Then it moves into another resonant frequency and the nodes are located in different spots on the sheet, resulting in a different pattern to the rice. There does exist a lowest possible frequency for resonance, presumably the really simple pattern you first see, but there is no theoretical max to the highest resonance state, just technical limits to how fast you can drive the sheet.