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.
GENERAL SCIENTIFIC SUMMARY Introduction and background. Mixing and segregation in either natural or industrial slurries is a key issue: segregation is either promoted in separation processes or, when homogeneity is required (as in cement slurries), it is avoided. Fluidized beds have been used for a long time to either mix monodisperse (one size) suspensions, or segregate bidisperse (two sizes) suspensions. A constant upward mean flow, imposed in the fluidization column, produces a stationary suspension at a concentration corresponding to a balance between the buoyancy of the particles and the drag produced by the flow. In the bidisperse case, however, the difference in settling velocity between small and large particles induces a segregation process, which ends with a stationary stratified state in which the small particles lie on top of the large. For the simple case of non-Brownian, inertialess spherical particles, the dynamics of the segregation, and the existence of a segregation inhibition when particle sizes are too close, remain open issues which are addressed in this experimental investigation.
Main results. In this paper, thanks to the measurement of sound attenuation along the column, we follow the segregation process obtained for large size ratios. However, for a size ratio below 1.41, the long-time fluidization results in an unexpected quasi-periodic intermittency of segregation and mixing phases.
Wider implications. Bidisperse macroscopic suspensions are a paradigm of athermal binary systems and their understanding should enhance the statistical description of such systems.
Figure. Sketch of the fluidization of a bidisperse suspension, color-coded axial composition, and typical spatio-temporal composition diagrams for the segregation (top right) and intermittency regimes (bottom right).