Chemical Engineering Science, Vol.56, No.9, 2967-2977, 2001
Microstructure evolution and theological responses of hard sphere suspensions
In the present study, the microstructural transitions of concentrated 'hard-sphere' suspensions under a simple shear flow were investigated by measuring the shear viscosity and flow-induced dichroism. Monodisperse silica particles of two different sizes, 260 and 545 nm in diameter, were prepared by the so-called modified Stober method. The monodisperse particles were coated with 3-(trimethoxysilyl)propyl methacrylate (MPTS) to enhance the dispersion stability at high particle volume fractions (phi) up to phi = 0.55. The particles were dispersed in a refractive-index matching solvent, tetrahydrofurfuryl alcohol, in which the van der Waals dispersion forces were diminished. The smaller particle suspension exhibited a smooth shear thinning up to phi = 0.55 within our shear rate window, independently of the surface modification with the silane coupling agent MPTS. Meanwhile, for the larger silica particle suspension, the viscosity shear thinned at low shear rates and shear thickened at high shear rates when phi exceeded about 0.5. The surface modification enhanced its dispersion stability and slightly decreased the sheer viscosity. Finally, the flow-induced dichroism from light passed in the flow-gradient direction probed the order-disorder transition effectively for the larger particle suspensions, such as disappearance of hexagonally ordered layered structure and formation of particle clustering.