Structure, density, and velocity fluctuations in quasi-two-dimensional non-Brownian suspensions of spheres

Non-Brownian sedimenting suspensions exhibit density and velocity fluctuati ons. We have performed experiments on a quasi-two-dimensional counter-flow stabilized suspension of 2000 spherical particles, namely a liquid-solid fl uidized bed in a Hele-Shaw cell. This two-dimensional suspension displays a uniform concentration but the particle radial distribution function and th e fluctuations of the particle number in a subvolume of the suspension sugg est that the microstructure is far from being random. We have also measured the velocity fluctuations of a test particle and the fluctuations of the m ean particle velocity in a subvolume. It happens that the relation between velocity and concentration fluctuations in a subvolume can be deduced from a balance between buoyancy and parietal friction forces. (C) 2000 American Institute of Physics. [S1070-6631(00)02305-9].