Shear-thickening rheology of concentrated nanoparticle suspensions containing non-adsorbing polymer

The additive manufacturing technique of direct-ink-writing can be used to fabricate custom and precise structures from ceramic nanoparticle suspensions for applications in electronics and biomedical industries, with polymers often added as viscosity modifiers. Understanding how polymer addition affects concentrated suspension rheology, and in particular the onset of shear-thickening, is crucial for the design of 3D-printable polymer-ceramic suspensions. Here, we examine the rheology of aqueous alumina nanoparticle suspensions with and without addition of non-adsorbing polyvinylpyrrolidone (PVP). For PVP-free suspensions, the viscosity and yield stress both increase as the alumina loading increases from 55 to 57.5 vol%. Discontinuous shear-thickening is observed at high shear rates and the onset stress of shear-thickening decreases slightly with increasing alumina loading. Addition of 0.5-5 vol% PVP to a 55 vol% alumina suspension also causes large increases in both viscosity and yield stress. Interestingly, in this case the onset stress of shear-thickening increases with polymer addition. We relate the overall suspension rheology to the polymer conformation in the suspending medium, demonstrating how polymer addition can extend the processing window during extrusion.