Suspensions of carbon black are used to manufacture everyday products like rubber tires, inks and paints. They also play a crucial role as conductive additives in battery electrodes and catalyst support in fuel cells. Carbon black suspensions can exhibit shear-dependent behaviors that may lead to undesirable forms of agglomeration.
To better understand such behaviors, Hipp et al. investigated the structural evolution of agglomerates in carbon black suspensions when subject to high shear rates. Their experiments on suspensions with varying properties revealed striking similarities in hierarchical microstructural evolution, yield stress and shear-thinning viscosity. The observed universal behavior allowed them to create a master curve for the microstructure, as well as a relation between the bulk viscosity, shear intensity and microstructure.
The researchers studied eight different samples with a range of carbon black primary particle structures, suspending fluids and concentrations. Direct measurements of shear-induced microstructure during a range of shear rates were performed at the NIST Center for Neutron Research using rheometry combined with very small angle neutron scattering and ultra-small angle neutron scattering techniques.
The experimental results agree with quantitative numerical predictions for the self-similar agglomerate break-up process, which was found to depend on suspension properties and flow strength. The dimensionless modified Mason number used as the independent variable includes the bulk stress that increases as more particles are added to the suspension.
The work falls into a larger theme in the field of moving beyond simple structure-property relationships to looking at time-dependent and history-dependent rheological behavior. The researchers hope that the methodology of the current study will be applied to better understand a broader range of complex fluids including gels, silicas and polymers.
Source: “Direct measurements of the microstructural origin of shear-thinning in carbon black suspensions,” by Julie B. Hipp, Jeffrey J. Richards, and Norman J. Wagner, Journal of Rheology (2021). The article can be accessed at http://doi.org/10.1122/8.0000089.
