Unraveling the Role of Frictional Contacts and Particle Orientational Order During Shear-thickening in Suspensions of Colloidal Rods

There is now convincing evidence that inter-particle frictional contacts are essential for observing shear-thickening in concentrated suspensions of compact particles. While this has inspired many strategies to tailor the rheology in these systems, in the more general case, viz-a-viz suspensions of anisotropic particles, the mechanism of shear-thickening remains unclear. Here through simultaneous measurements of the bulk viscosity and the first Normal stress difference, we show that strong shear thickening in suspensions of colloidal rods is accompanied by large positive normal stresses, indicating the formation of a system-spanning frictional contact network. We also find that flow in the shear-thickening regime is unsteady and shows a rather rich time-dependence. By carrying out single particle-resolved confocal rheology measurements, we provide compelling evidence that this rheological chaos arises from a strong coupling between the imposed flow and particle orientational order. Building on these observations, we designed colloidal rods with temperature-tunable tribological properties and demonstrate the feasibility of achieving in-situ control over suspension rheology. These findings show that the interplay between orientational order and frictional interactions plays a critical role in the shear thickening of dense suspensions of colloidal rods.