  In this work, the Z-average, effective, apparent diffusion coefficients and
their poly-dispersity indexes were investigated for dilute poly-disperse
homogeneous spherical particles in dispersion where the Rayleigh-Gans-Debye
approximation is valid. The results reveal that the values of the apparent and
effective diffusion coefficients at a scattering angle investigated are
consistent and the difference between the effective and Z-average diffusion
coefficients is a function of the mean particle size, size distribution and
scattering angle. For the small particles with narrow size distributions, the
Z-average diffusion coefficient can be got directly at any scattering angle.
For the small particles with wide size distributions, the Z-average diffusion
coefficient should be measured at a small scattering angle. For large
particles, in order to obtain a good approximate value of Z-average diffusion
coefficient, the wider the particle size distribution, the smaller the
scattering angle that the DLS data are measured. The poly-dispersity index of
the effective diffusion coefficient at a scattering angle investigated is
consistent with that of the Z-average diffusion coefficient and without
considering the influences of noises, the difference between the
poly-dispersity indexes of the Z-average and apparent diffusion coefficients is
determined by the mean particle size, size distribution and scattering angle
together.
