Shear thickening in steady and superposition flows effect of particle interaction forces

The phenomenon of shear thickening has been studied in sterically stabilize d colloidal suspensions. Temperature has been used to vary the solvent qual ity of the suspending medium for the grafted polymer in the stabilizer laye r. A decrease in temperature causes the polymer coat to shrink. This result s in a lower viscosity in the shear thinning region but also in a lower cri tical shear rate and shear stress at the onset of shear thickening. The cri tical shear stress increases with increasing particle volume fraction, at l east for the volume fractions studied here. In large amplitude oscillatory flow, strain hardening for both storage and loss moduli sets in at the same peak shear rate that causes shear thickening in steady state flow. Paralle l superposition also has been used to probe the effect of the stabilizer la yer. The frequency dependence of the superposition moduli changes drastical ly once shear thickening sets in. Surprisingly, this: produces a simple and constant shape for the moduli-frequency curves with relaxation times that become nearly independent of the shear rate. (C) 2001 The Society of Rheolo gy.