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 (phi = 0.05-0.12) and shear rates (gamma = 10(1)-10(3) 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 phi was increased above 0.1. Relative viscosity (eta(r)) of the suspensions followed an exponential form with phi, i.e., eta(r) = 13.47e(35.98)phi, revealing a pronounced increase in the degree of particle interactions as phi increased. Fractal dimension (D-f) was estimated from the suspension yield-stress (tau(gamma)) and phi dependence, and was determined as D-f similar to 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 (phi(m)) of the suspensions was determined as phi(m) = 0.146. This relatively low value of phi(m) (compared with the random close packing of monosized particles, phi(m) similar to 0.64) partially vindicated the existence of a porous, three-dimensional aggregate network of interconnected nanoparticles in the carrier liquid. (C) 2003 Elsevier Science B.V. All rights reserved.