Segregation and periodic mixing in a fluidized bidisperse suspension

Abstract

We address the issue of segregation in bidisperse suspensions of glass beads, by using a liquid fluidized bed in the inertialess regime and an acoustic technique for acquiring the axial composition along the column. Fluidization balances the buoyancy of the particles by a constant uniform upward flow, and therefore enables long-time experiments. From the analysis of the transient segregation fronts, we have collected precise measurements on the sedimentation velocities of small and large beads, Us and Ul, in homogeneous suspensions at the same volume fraction, , for both the bead species, and for different size ratios, 1.13⩽λ⩽1.64, and solid concentrations, . Our measurements provide evidence for a difference in the sedimentation velocities, Us and Ul, over all the ranges of λ and covered. These results make one expect that a long-term fluidization should then result in a stationary segregated state, which was indeed always obtained for large enough particle size ratios, λ⩾1.43. However, at high concentration and for particles of close sizes, λ⩽1.41, we observed a surprising pseudo-periodic intermittency of slow segregation and quick mixing phases. The intermittency time is much longer than the batch sedimentation time and becomes noisy at very high concentration, for which metastable states have been observed. The origin of the mixing destabilization remains an open issue, but we note however that the domain of occurrence, λ⩽1.41, also corresponds, in our experiments, to a continuous size distribution of the particles.