Particle Motions in a Gas-Fluidized Bed of Sand

We report measurements by diffusing-wave spectroscopy of particle motions in a gas-fluidized bed. The homogenous state of the bed known as the uniformly fluidized state is actually a weak solid in which particles are at rest. The only truly fluid state is an inhomogenous mixture of gas bubbles and of liquid regions in which microscopic particle dynamics are shown to be collisional. The motion of macroscopic bubbles is the source of particle motions in the bed. Measurements of mean free paths, collision rates, and velocity fluctuations (the ``granular temperature'') show that dissipation occurs nonuniformly in the medium.