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 l arge 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. Th e medium was ethyleneglycol, possessing a refractive index very close to that of the used silica particles. The medium reduced effectively i nterparticle potentials due to dispersion forces, and it provided bimo dal suspensions with only the hard core interparticle potential. The t otal weight fraction of the suspended particles was kept at 52 wt %, w hile 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 bi modal suspensions showed viscoelastic behavior with a frequency depend ence similar to that of a unimodal (monodisperse) suspension, which wa s attributed to the contribution of Brownian motion of suspended parti cles. The zero-shear viscosities (eta(0)) of the bimodal suspensions s howed minima at X-S values dependent on r(L /) r(S), whereas the high frequency limiting viscosities showed no obvious minima. A simple mode l is proposed to interpret these viscoelastic features in the bimodal suspensions. In the model, we assumed that in the suspension a Brownia n motion of a hypothetical particle with an average radius governed th e whole viscoelastic features.Values of eta(0) predicted by the model and experiments agreed fairly well. (C) 1998 The Society of Rheology.