Long-range correlations in sedimentation

Using particle image velocimetry we have measured the sedimentation dynamics of non-Brownian colloidal spheres over a wide range of low concentrations (phi less than or equal to 0.05) and sample cell sizes W. Fluctuations in the settling velocity show universal large-scale finite-range correlations, in the form of swirls, which depend simply on volume fraction and particle radius and follow [delta VzV-(0)delta V-z(z)](1/2) similar or equal to 2.0V(sed)phi(1/3) exp(-z/2 xi), where xi = 20a phi(-1/3) In turn, the predicted divergence of the velocity variance with increasing cell size W is cut off in a universal way as W exceeds the swirl size and is well represented by [delta V-z(2)(0)](1/2) = 2.01(sed)phi(1/3) [1-exp(-W/60a phi(-1/3))].