Self-diffusion in a gas-fluidized bed of fine powder

We have investigated the self-diffusion in a stable gas-fluidized bed of fine powder. Two regimes have been observed: for gas velocities v(g) above the minimum fluidization velocity v(m) and below a critical gas velocity v(c) smaller than the minimum bubbling velocity v(b) the powder does not mix. Experimental measurements show the existence of yield stresses in this regime which are responsible for the static behavior of the bed. For v(g)>v(c) the yield stress vanishes; the bed behaves like a fluid and displays a diffusive dynamics. In this region we have found that the diffusion coefficient D increases with gas velocity until the bed expansion approaches its maximum value.