The mobility of fine particles in uniform gas-fluidized beds.

We have investigated the mobility of the particles in a stable gas-fluidized bed. When the gas velocity upsilon (g) is reduced from an initial value in the free bubbling range to a value larger than the minimum fluidization velocity upsilon (min) and smaller than a certain critical value upsilon (c). the fluidized powder bed behaves as a weak solid. In this regime we have measured a non-vanishing tensile strength of the fluidized bed. For gas velocities above upsilon (c) the fluidized bed looses its mechanical strength and powder particles suspended in the fluidizing gas exhibit a diffusive dynamics due to the fluctuations on their motion. We have obtained an effective diffusion coefficient D as a function of the fluidizing gas velocity. We have found that D increases as the gas velocity is increased. For gas velocities close to the minimum bubbling velocity the this effective diffusion coefficient remains constant.