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 (φ=0.05–0.12) and shear rates (γ=101–103 s−1). 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 φ was increased above 0.1. Relative viscosity (ηr) of the suspensions followed an exponential form with φ, i.e., ηr=13.47e35.98φ, revealing a pronounced increase in the degree of particle interactions as φ increased. Fractal dimension (Df) was estimated from the suspension yield-stress (τy) and φ 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 (φm) of the suspensions was determined as φm=0.146. This relatively low value of φm (compared with the random close packing of monosized particles, φm∼0.64) partially vindicated the existence of a porous, three-dimensional aggregate network of interconnected nanoparticles in the carrier liquid.