Effect of ammonium polyacrylate on dispersion and rheology of aqueous ITO nanoparticle colloids

Dispersion of indium tin oxide (ITO) nanoparticles in water is examined via rheological characterizations. A pronounced viscosity reduction exceeding 99% at a shear-rate of 100 s−1 resulted when an ammonium polyacrylate (NH4PA) of 0.5–2 wt.% was introduced into the aqueous nanoparticle colloids. Zeta-potential measurement reveals that the NH4PA molecules adsorb preferentially on the nanoparticle surface, providing a sufficient electrosteric repulsion between particle surfaces so that the flow resistance was reduced. Suspension rheology resembles that of the Newtonian flow over a shear-rate (γ˙) range 10 to ∼500 s−1. The colloidal rheology then changes into a shear-thickening flow as γ˙ was further increased. A maximum solids concentration (ϕmax) at which the nanoparticle suspensions changes from a liquid-like form into a “solid” bulk is determined as ϕmax = 0.34 at γ˙=100 s−1. This low value of ϕmax suggests that the van der Waals attraction involved in the nanoparticle suspensions eventually prevails over all other interparticle and hydrodynamic forces when the interparticle separation is reduced to a critical level, despite the presence of the NH4PA surfactant molecules.