Size segregation and particle velocity fluctuations in settling concentrated suspensions

Abstract

16 pagesInternational audienceWe investigate the sedimentation of concentrated suspensions at low Reynolds numbers to study collective particle effects on local particle velocity fluctuations and size segregation effects. Experiments are carried out with polymethylmetacrylate (PMMA) spheres of two different mean diameters (190 and 25 μm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labelled PMMA spheres and a particle image velocimetry method. We also report measurements of the interstitial fluid pressure during settling. Experiments show that size segregation effects can occur during the sedimentation of concentrated suspensions of either quasi-monodisperse or bidisperse spheres. Size segregation is correlated to the organi-sation of the sedimentation velocity field into vortexlike structures of finite size. A loss of size segregation together with a significant decrease of the fluid pressure gradient in the bulk suspension is observed when the size of vortex-like structures gets on the order of the container size. However, the emergence of channels through the settling zone prevents a complete loss of size segregation in very concentrated suspensions

Experimental investigation of batch sedimentation of concentrated bidisperse suspensions

“…In the upper layer of the sediment where only small particles are present, the ϕ* is found to be approximately equal to 0.60 whatever the size ratio and is below the random close packing limit (ϕ* = 0.64), and it approaches the random loose packing limit (ϕ* = 0.56) that is representative of gravitational packing of monodisperse suspensions [28][29][30]. This finding is in agreement with experimental data already reported by Snabre et al [26]. In any case, although the impact of the sedimentation type cannot be easily highlighted by analyzing the sediment constitution, we show how the deposit structure does capture the history of the sedimentation process and of course depends on the early stages of sedimentation before a complete phase separation.…”

“…However, in the last few years, several modern techniques have been used to determine the time variation of concentration profiles of sedimenting concentrated suspensions. The method of Laser Induced Fluorescence has been recently demonstrated to be successful in following each particle population and was used to study velocity fluctuations [26].…”

Long-range orientational order in two-dimensional microfluidic dipoles

“…As in the 2D droplet ensembles reported here, also in gravitational sedimentation of particles in 2D and 3D, the velocity variance decreases at high densities 2,12,26 . However, so far this generic decrease has lacked a first-principles theoretical explanation.…”

Fluctuations in Particle Sedimentation

“…The inset of Figure 4.6b shows results for correlation functions obtained by Snabre et al [33] in both the vertical (z) and horizontal (x) directions. As is often found, the correlation functions decay nearly exponentially with distance, and simple fits yield characteristic correlation decay lengths .…”