Self-propelled particles can break out of shear thickening.
Dense suspensions of frictional grains shear-thicken: above a critical stress, viscosity jumps as particles form load-bearing frictional contact networks. This is well understood — the contacts multiply, the network percolates, and the suspension jams. The standard picture is entirely passive: apply more stress, get more contacts, get higher viscosity.
Singh et al. (arXiv:2603.09620) add self-propulsion to each grain and observe “dethickening” — a viscosity reduction at stresses where the passive system would be maximally thickened. The mechanism: self-propulsion introduces isotropic dynamics that compete with the flow-driven, anisotropic formation of frictional contacts. Active particles jitter in all directions. This random motion disrupts the aligned contact chains that support the stress. The network that would otherwise percolate is continually dismantled by the particles' own activity.
The degree of dethickening is tunable. A dimensionless active stress — the ratio of self-propulsion force to imposed flow stress — controls the transition. At low activity, the system behaves passively: shear-thickened, jammed, viscous. At high activity, the contacts can't form fast enough to survive the isotropic disruption, and the viscosity drops back to a fluid-like state.
The authors show that their data obey a recently proposed universal scaling framework that encompasses multiple routes to dethickening — vibration, temperature, and now activity. The implication: dethickening isn't a phenomenon specific to active matter. It's a general consequence of any mechanism that adds isotropic fluctuations to a system trying to build anisotropic contact networks. The contact network is a structure maintained against noise. Add enough noise, from any source, and the structure dissolves.
Singh et al., "Tunable shear thickening in active non-Brownian suspensions," arXiv:2603.09620 (2026).