Rheological behavior and stability of concentrated silica suspensions

In the present article, the rheological behavior and phase stability of con centrated silica suspensions were investigated experimentally by examining the effects of particle size and temperature. The silica particles were sta bilized by adsorption of a silane coupling agent, gamma-methacryloxypropyl triethoxy silane (MPTES). The MPTES-coated silica particles behaved like ha rd spheres and exhibited the dispersion stability in tetrahydrofurfuryl alc ohol which was used as a refractive-index matching solvent. For a monodispe rse suspension, the limiting viscosities at high shear rates were correlate d satisfactorily with the Krieger-Dougherty equation until the particle vol ume fraction phi reached 0.45 above which the limiting high-shear-rate visc osities did not exist. The highly concentrated monodisperse suspensions abo ve phi = 0.50 displayed the rapidly shear thinning viscosity at low shear r ates and underwent the shear thickening at high shear rates. Specifically, the onset of shear thickening shifted to a higher shear rate either as temp erature rose or as the suspension became stable. The dichroism from light p assed in the flow-gradient direction probed the most effectively the order- disorder transition such as disappearance of hexagonally ordered layered st ructure and formation of particle clustering, which caused shear thickening at high shear rates. The abrupt change in dichroism caused by the breakdow n of hexagonally layered structure occurred almost 1 decade before the onse t of shear thickening. Finally, the bimodal suspensions prepared here posse ssed the reduced shear viscosity and stress in comparison with the monodisp erse suspensions of identical volume fraction. The degree of the viscosity reduction became conspicuous for concentrated suspensions usually above phi = 0.40. (C) 1999 The Society of Rheology. [S0148-6055(99)01105-0].