  A study on the spatial organization and velocity fluctuations of non Brownian
spherical particles settling at low Reynolds number in a vertical Hele-Shaw
cell is reported. The particle volume fraction ranged from 0.005 to 0.05, while
the distance between cell plates ranged from 5 to 15 times the particle radius.
Particle tracking revealed that particles were not uniformly distributed in
space but assembled in transient settling clusters. The population distribution
of these clusters followed an exponential law. The measured velocity
fluctuations are in agreement with that predicted theoretically for spherical
clusters, from the balance between the apparent weight and the drag force. This
result suggests that particle clustering, more than a spatial distribution of
particles derived from random and independent events, is at the origin of the
velocity fluctuations.
