Motility-induced shear thickening in dense colloidal suspensions

Phase transitions and collective dynamics of active colloidal suspensions are fascinating topics in soft matter physics, particularly for out-of-equilibrium systems, which can lead to rich rheological behaviour in the presence of steady shear flow. In this study, the role of self-propulsion in the rheological response of a dense colloidal suspension is investigated by using particle-resolved simulations. First, the interplay between activity and shear in the solid to the liquid transition of the suspension is analysed. While passive suspension shows a solid-like behaviour, turning on particle motility fluidises the system and, at low self-propulsion, the suspension behaves as a shear-thinning fluid. Increasing the self-propulsion of the colloids induces a crossover from a shear-thinning to a shear-thickening behaviour, which we attribute to clustering in the suspensions induced by motility, a general phenomenon that occurs close to motility-induced phase separation (MIPS) [I. Buttinoni, et al., Physical Review Letters. 110, 238301 (1-5) (2013)]. This novel behaviour of motility-induced shear thickening (MIST) can be used to tailor the rheological response of colloidal suspensions.