Measurement of the hydrodynamic fractal dimension of aggregating polystyrene spheres

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Abstract
Slow (reaction-limited) aggregation of small polystyrene spheres in aqueous suspension was started by the addition of salt and was monitored by conventional and dynamic light scattering at small scattering angle. The average aggregation number and hydrodynamic radius obtained from these measurements were related through a ‘hydrodynamic’ fractal dimension d h. Once the average aggregate size became larger than about 1 μm we also measured, from the angular dependence of the scattered intensity, a ‘structural’ fractal dimension dm . The two dimensions so obtained agreed within experimental error, implying a proportionality of structural and hydrodynamic radii; their values, about 2·08, are similar to those found by other workers for slow aggregation in three dimensions.