Settling statistics of hard sphere particles

Direct imaging of settling, non-Brownian, hard sphere, particles allows mea surement of particle occupancy statistics as a function of time and samplin g volume dimension. Initially random relative particle number fluctuations, <N-2 - <N > (2)>/<N > = 1,become suppressed, anisotropic, and <N > depende nt. Fitting to a simple Gaussian pair correlation model suggests a minute l one ranged correlation leads to strong if not complete suppression of numbe r fluctuations. Calflisch and Luke predict a divergence in velocity fluctua tions with increasing sample volume size based on random (Poisson) statisti cs. Our results suggest this is not a valid assumption for settling particl es.