Rheology and colloidal structure of aqueous TiO2 nanoparticle suspensions

Rheological behavior and suspension structure of anatase titanium dioxide (TiO2) nanoparticles dispersed in pure water have been investigated over a range of volumetric solids concentrations (*q =0.05-0.12) and shear rates (*g=101-103 s-l). The nanoparticle suspensions generally exhibited a pseudoplastic flow behavior, indicating an existence of particle aggregations in the liquid medium. The suspensions became apparently thixotropic as *q was increased above 0.1. Relative viscosity (*hr) of the suspensions followed an exponential form with *q, i.e., *hr = 13.47e35.98*q, revealing a pronounced increase in the degree of particle interactions as *q increased. Fractal dimension (Df) was estimated from the suspension yield-stress (ry) and *q dependence, and was determined as Df#~1.46-1.78 for the flocculated nanoparticle suspensions. This suggested that the suspension structure was probably dominated by the diffusion-limited cluster-cluster aggregation, due mostly to the strong attractions involved in the interparticle potentials. Maximum solids loading (*qm) of the suspensions was determined as *qm= 0.146. This relatively low value of *qm (compared with the random close packing of monosized particles, *qm #~ 0.64) partially vindicated the existence of a porous, three-dimensional aggregate network of interconnected nanoparticles in the carrier liquid.