Rheology of 3D printable ceramic suspensions: effects of non-adsorbing polymer on discontinuous shear thickening

Concentrated suspensions of particles at volume fractions (ϕ) ≥ 0.5 often exhibit complex rheological behavior, transitioning from shear thinning to shear thickening as the shear stress or shear rate is increased. These suspensions can be extruded to form 3D structures, with non-adsorbing polymers often added as rheology modifiers to improve printability. Understanding how non-adsorbing polymers affect the suspension rheology, particularly the onset of shear thickening, is critical to the design of particle inks that will extrude uniformly. In this work, we examine the rheology of concentrated aqueous suspensions of colloidal alumina particles and the effects of adding non-adsorbing polyvinylpyrrolidone (PVP). First, we show that suspensions with ϕ_alumina = 0.560-0.575 exhibited discontinuous shear thickening (DST), where the viscosity increased by up to two orders of magnitude above an onset stress (τ_min). Increasing ϕ_alumina from 0.550 to 0.575 increased the viscosity and yield stress in the shear thinning regime and decreased τ_min. Next, PVP was added at concentrations within the dilute and semi-dilute non-entangled regimes of polymer conformation (ϕ_PVP = 0.005-0.050) to suspensions with constant ϕ_alumina = 0.550. DST was observed in all cases and increasing ϕ_PVP increased the viscosity and yield stress. Interestingly, increasing ϕ_PVP also increased τ_min. We posit that the free PVP chains act as lubricants between alumina particles, increasing the stress needed to induce thickening. Finally, we demonstrate through direct comparisons of suspensions with and without PVP how non-adsorbing polymer addition can extend the extrusion processing window due to the increase in τ_min.