Tunable shear thickening in active frictional non-Brownian suspensions

We study tunable shear thickening in active suspensions of non-Brownian, repulsive, frictional grains using particle-based simulation, finding that activity augments the rheology beyond the friction-mediated shear thickening paradigm. Specifically, up to a crossover activity strength, increasing particle self-propulsion drives a viscosity-reducing 'dethickening' of the system at large stresses, where the material would otherwise be in a thickened, highly viscous state. Above this crossover activity strength, the system enters a thickening regime. Self-propulsion introduces additional isotropic dynamics to the particles, which compete with the flow-driven formation of frictional contacts. The degree of dethickening can thus be tuned by varying a suitably-defined dimensionless active stress that quantifies this competition. Recognising the parallels between self-propulsion and other contemporary routes to dethickening, we demonstrate that our data, within the dethickening regime, are consistent with a recently proposed universal scaling framework, offering a route to tuning the rheology of dense suspensions. Moreover, using the obtained scaling relation, we infer how the shear-jamming and shear-thickening regimes can be tuned by choosing an appropriate self-propulsion strength, and partially validate these inferences with targeted simulations.