The Rheology of Bimodal Mixtures of Colloidal Particles with Long-Range, Soft Repulsions

Osmotic pressure and elastic moduli of bimodal suspensions of particles experiencing long-range, soft repulsions were measured. At fixed osmotic pressure, the total suspension volume fraction, φ, varies linearly as the mixing ratio φ_l/φ is increased from 0 to 1. Here φ_lis the volume fraction of large particles based on total suspension volume. This result suggests the suspensions studied here are phase separated into domains containing primarily small and primarily large particles and is not expected for hard sphere suspensions where, at fixed osmotic pressure, φ passes through a maximum as φ_l/φ is increased. Elastic moduli are well described by a model based on a composite microstructure where the domains of pure large and small particles must have the same osmotic pressure which fixes the local particle volume fraction and hence the elastic modulus in each phase. The existence of phase separation is supported by electron micrographs taken on samples prepared by rapidly drying suspensions with volume fractions near 0.6.