Viscoelastic behavior of bimodal suspensions

The dynamic viscoelastic behavior of suspensions with bimodal particle radius distributions was examined varying the ratio of the radii of large and small particles (r(L) / r(S)) up to 3.3. Monodisperse silica spherical particles with radii ranging from 65 to 215 nm were used. The medium was ethyleneglycol, possessing a refractive index very close to that of the used silica particles. The medium reduced effectively interparticle potentials due to dispersion forces, and it provided bimodal suspensions with only the hard core interparticle potential. The total weight fraction of the suspended particles was kept at 52 wt %, while the weight composition of the small particles (X-S) to the total particulate mass was increased from 0 to 1 with a 0.2 interval. The bimodal suspensions showed viscoelastic behavior with a frequency dependence similar to that of a unimodal (monodisperse) suspension, which was attributed to the contribution of Brownian motion of suspended particles. The zero-shear viscosities (eta(0)) of the bimodal suspensions showed minima at X-S values dependent on r(L /) r(S), whereas the high frequency limiting viscosities showed no obvious minima. A simple model is proposed to interpret these viscoelastic features in the bimodal suspensions. In the model, we assumed that in the suspension a Brownian motion of a hypothetical particle with an average radius governed the whole viscoelastic features.Values of eta(0) predicted by the model and experiments agreed fairly well. (C) 1998 The Society of Rheology.