The nonequilibrium aggregation structure of primary particles in colloidal bidispersions is investigated at high volume fractions by Brownian dynamics simulations. It is found that introducing limited different sized particles in the monodispersion can obviously affect the short-range structures of primary particles. In a bidispersion, fractal dimension of aggregates, only consisting of primary particles, increases with increasing the size difference in the long-range scale. The structure factor S(q) of aggregates, obtained from the particle correlation function g(r), suggests that fractal structure disappears when the primary particles become not "primary" in volume fraction.
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