Sigmoidal temporal increase in shear viscosity of titania–toluene–ethanol colloidal suspensions

Abstract

The shear viscosity of toluene-based TiO2 nanoparticle/poly(vinyl acetate) suspensions initially increased gradually, then increased steeply, and finally levelled off under shearing in the presence of ethanol as a polar additive. This non-linear response followed a sigmoidal function with a characteristic rate constant for a given composition. The rate constant reached a maximum at a critical ethanol concentration, resulting in maxima of the final viscosity and storage modulus that have not been previously reported for non-aqueous macroscopically miscible systems. This viscosity anomaly was attributed to the preferential adsorption of the polar solvent and the resulting alcohol-clustering-induced particle networking in the non-polar medium. The temporal variations in viscosity were reproduced using a proposed kinetic model that newly considered inter-particle bonding assisted by ethanol adsorption. These results elucidate the transient rheological responses of suspensions containing polar/non-polar solvents for coating applications.