Self-Organization and Chaos in a Fluidized Bed

We present experimental evidence that a complex system of particles suspended by upward-moving gas can exhibit low-dimensional bulk behavior. Specifically, we describe large-scale collective particle motion referred to as slugging in an industrial device known as a fluidized bed. As gas flow increases from zero, the bulk motion evolves from a fixed point to periodic oscillations to oscillations intermittently punctuated by ``stutters,'' which become more frequent as the flow increases further. At the highest flow tested, the behavior becomes extremely complex (``turbulent'').