Contrasting the flow response of aqueous dispersions of colloidally stable polymer particles with dispersions controlled only by hydrodynamic interactions, we find that the hydrodynamic interactions are still a major factor, but colloid chemical forces can modify the response in important ways. For example, at solids levels between 55 and 70 vol %, a bimodal size distribution can be used to greatly enhance the flow properties of the dispersion. Minimum viscosity levels are achieved by close attention to the size ratio and the weight ratio of the two sizes, as well as to the absolute size of the particles. Size and weight ratios are known to have an important effect on hydrodynamic interactions, but the absolute size of the particles becomes a factor whenever colloid chemical forces come into play.

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