Structure-property relationships of sheared carbon black suspensions determined by simultaneous rheological and neutron scattering measurements

Carbon black suspensions exhibit complex, shear-dependent macroscopic properties that are a consequence of the state of the suspension microstructure. In this work, the shear-induced microstructure of a model, reversible suspension of conductive carbon black in propylene carbonate is measured using simultaneous steady shear rheology and small angle neutron scattering. These experiments provide microstructural evidence for a bifurcation in the rheological properties. We show that the demarcation line for this bifurcation is the inverse Bingham number, Bi^-1, which relates the magnitude of the stress response to an applied shear rate to the yield stress of the presheared suspension. At high shear rates where Bi^-1 > 1, the suspension flows homogeneously and exhibits a thixotropic response that arises due to the self-similar breakdown of agglomerates with increasing shear rate. Conversely, at low shear rates where Bi^-1 < 1, the applied shear drives the densification and growth of these agglomerates. This densification process leads to a gravitationally driven instability resulting in an inhomogeneous volume fraction distribution along the height of the geometry that is a function of both time under shear and shear rate. Under these shear conditions, the suspension exhibits apparent rheopexy, or antithixotropy, where a significant decline in the viscosity is observed with a step down in shear rate. The unique microstructural measurements presented here reconcile many observations in the literature regarding carbon black suspensions, including an apparent shear-thickening behavior, tunability of both the yield stress and elasticity through shear history, and transient macroscopic properties.