Rheology of Aqueous Suspensions of Polystyrene Particles with Bimodal Radius Distribution

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Rheological behavior was examined for aqueous suspensions of a mixture of monodisperse polystyrene (S) particles having different radii, 42 nm and 103 nm. The bare volume fraction of the particles, φ, was almost identical in all suspensions examined (φ = 0.42−0.43), and the mixing ratio of the small particles, ws, was varied. The particles had an electrostatic shell due to the surface charges, and the shell thickness was different for the small and large particles in respective unimodal suspensions. Thus, the viscoelastically effective volume fraction φeff (including the shell volume) of the particles, being larger than φ, changed with ws. In the linear viscoelastic regime, the suspensions exhibited terminal relaxation attributable to Brownian motion of the particles, and the terminal relaxation time τ and zero-shear viscosity η0 first decreased and then increased with increasing ws. These viscoelastic features were compared with predictions of the Shikata-Niwa-Morishima (SNM) model considering the Brownian motion of hypothetical unimodal particles with a radius being equal to an average of the radii of the large and small particles. The ws dependence of τ and η0 of the aqueous S suspensions was well described by this model, given that the change of φeff with ws was accounted. Under steady flow, the suspensions of the S particles exhibited shear thinning of the viscosity. This thinning, attributable to nonlinearity of the Brownian stress, was less significant for the bimodal suspensions (in particular for the suspension with w s = 0.25) than for the unimodal suspensions (ws = 0 and 1). This difference resulted from a difference of the strain γcol required for particle collision under flow: γcol was the largest for the bimodal suspension with ws = 0.25 thereby inducing the weakest thinning in this suspension. In fact, the unimodal and bimodal suspensions exhibited an universal relationship between the normalized viscosity and normalized strain γ / γcol (with γ being the strain effectively imposed through the flow) irrespective of the ws value.
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Rheology of Aqueous Suspensions of Polystyrene Particles with Bimodal Radius Distribution

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