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

We have investigated the self-diffusion in a stable gas-fluidized bed of fi ne powder. Two regimes have been observed: for gas velocities v(g) above th e 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. E xperimental measurements show the existence of yield stresses in this regim e 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 dif fusive dynamics. In this region we have found that the diffusion coefficien t D increases with gas velocity until the bed expansion approaches its maxi mum value.