NUMBER FLUCTUATIONS OF INTERACTING PARTICLES

The mean-square fluctuation in the number of colloidal particles (radius approximately 45 nm) in a small volume element ( approximately 8 mu m 3) of an aqueous dispersion was measured by photon-correlation laser light scattering. Both randomly distributed, non-interacting particles and those showing a 'liquid-like' spatial arrangement owing to long-range repulsive Coulombic interactions were studied. The magnitude of the reduced fluctuations in the latter case agreed with that predicted from the structure of the dispersion, which was determined independently from the angular dependence of the average scattered light intensity. This provides the first direct experimental verification of the fundamental Ornstein-Zernike relationship between numbers fluctuations and the pair distribution function g(r) in a system of interacting particles. Possible extensions of the experiment, including the measurement of four-particle correlations, are discussed briefly.