We show that the coupling between aggregation and sedimentation in colloidal suspensions gives rise to either cluster deposition or to settling of a gelled suspension, depending on the volume fraction of particles. In the latter case, we report novel settling properties which are interpreted in terms of the specific spatial structuring of the suspensions. PACS numbers: 82.70.Dd, 64.60.Cn, 66.90.+r, 82.70.Gg Recently, many works have been devoted to understanding aggregation phenomena in

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... l suspensions. When the rate of growth is controlled by the Brownian diffusion of clusters, it is now established that the diffusion limited cluster aggregation (DLCA) model provides a fairly good description of simulations and experiments [1] [2] [3] [4] . By contrast, investigations of situations where aggregation is due to the combined action of Brownian motion and an imposed external field like gravity are scarce [5, 6] . At short times, initially dispersed particles and clusters undergo Brownian motion, and aggregation is still correctly described by the DLCA model. Later on, the clusters may be large enough to settle under gravity, and sedimentation then alters the growth mechanism. In this Letter, we study a set of novel phenomena arising from the coupling between aggregation and sedimentation in colloidal dispersions. The purpose is twofold. First, we demonstrate that aggregating suspensions may undergo settling in two ways when the volume fraction of particles is varied: by cluster deposition or the collective settling of a gelled suspension. We present the physical origins of these two phenomena using a scaling approach which makes it possible to take into account the competition between cluster diffusion and sedimentation. Second, we show that the settling dynamics of the gelled suspensions can be explained in terms of the existence of a specific spatial structuring. The experiments reported in the following have been performed with aqueous suspensions of calcium carbonate particles (Socal U1 supplied by Solvay Co.). Calcium carbonate is dense, r 2.7 g͞cm 3 , so that settling takes place rapidly. The particles are 0.07 mm in diameter. Their surface charge is ruled by the concentration of calcium and carbonate ions in the suspending medium [7] [8] [9] . In the range of volume fractions investigated ͑10 24 , F , 3 3 10 22 ͒, the concentration of calcium ions remains nearly constant, about 2 3 10 24 mol͞l, and the pH increases slightly from 8.7 to 9.2. In these conditions, the charge borne by the particles is very small [7-9] and the net colloidal interaction is the van der Waals attraction without electrostatic repulsion. This result is supported by the observation that suspensions are not sensitive to ionic strength variations. The samples are prepared by adding weighed amounts of calcium carbonate powder to deionized water. Before undertaking an experiment, the clusters are fragmented and the colloidal particles dispersed by strong stirring for at least 100 h. We have checked the efficiency and the reproducibility of the dispersion state of the suspension by optical and electronic microscopy. After stirring, the colloidal particles are well dispersed and no cluster with a diameter exceeding 0.2 mm can be found in the suspension. The experimental cells are cylinders which are 12 mm in diameter and 70 mm high; they are completely filled with suspension and sealed carefully to avoid the existence of a meniscus on the top of the suspension. Experimentally, the origin of the time scale is taken when the suspensions are poured into the experimental cells. We have studied extensively the time evolution of the suspensions as a function of the volume fraction F and we have found three distinct behaviors. At low volume fraction, we observe cluster deposition. Just after the suspension has been poured into the experimental cell, it appears uniformly turbid but, after a time, it becomes lumpy and we can discern small aggregates of different sizes which are settling at different velocities. The largest clusters, which fall the fastest, sweep up the small ones underneath and grow the fastest. Gradually, the suspension becomes less and less turbid and, later on, we see perfectly well-defined clusters settling separately in a clear fluid. They deposit at the cell bottom and form a voluminous sediment which compacts slowly. At higher volume fractions, cluster deposition is replaced by collective settling. Aggregates no longer settle separately; instead, they form a gel very quickly, which fills in the whole experimental cell and settles under its own weight afterward. Now, a sharp interface separates the suspension settling underneath from the supernatant clear of particles above. This interface moves downward until it reaches its equilibrium height. Experimentally, we have found that the volume fraction F ‫ء‬ separating cluster deposition from collective settling is F ‫ء‬ Х 3 3 10 23 . Finally, there exists a volume fraction, F ‫ءء‬ Х 5 3 10 22 , above which the colloidal gel does not settle. The existence and the value of F ‫ءء‬ are related to the mechanical properties of the calcium carbonate gel at 1478 0031-9007͞ 95͞74(8)͞1478(4)$06.00