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Polydispersity probed by light scattering of secondary particles in controlled growth experiments of silica spheres

From dynamic light scattering measurements of diluted silica dispersions formed in controlled growth experiments we obtained effective diffusion coefficients De(K) which exhibit a pronounced maximum as a function of the scattering vector K. The maximum is due to the presence of particles much smaller than the grown particles and which are formed in a secondary nucleation. It appears that De(K) is a sensitive probe for the presence of the small particles. From both theory and experiment we conclude the following. (1) The maximum is a consequence of small particle scattering at a K value where the scattering of the large particles is at a minimum. (2) The magnitude of the maximum is a sensitive measure of polydispersity in size of the large particles. Similar conclusions are drawn from measurements of the second cumulant Q(K). This function provides a measure of the width of the distribution of diffusion coefficients. Its K dependence curve shows a very peculiar shape, due to the presence of the small parti...