Collective dynamics and rheology of confined active suspensions*

Similarly to their biological counterparts, suspensions of chemically active autophoretic swimmers exhibit nontrivial dynamics involving self-organization processes as a result of inter-particle interactions. Using a kinetic model for dilute suspensions of autochemotactic Janus particles, we analyse the effect of a confined pressure-driven flow on these collective behaviors and the impact of chemotactic aggregation on the effective viscosity of the active fluid. Four dynamic regimes are identified when increasing the strength of the imposed pressure-driven flow, each associated with a different collective behaviour resulting from the competition of flow- and chemically-induced reorientation of the swimmers together with the constraints of confinement. Interestingly, we observe that the effect of the pusher (resp. puller) hydrodynamic signature, which is known to reduce (resp. increase) the effective viscosity of a sheared suspension, is inverted upon the emergence of autochemotactic aggregation. Our results provide new insights on the role of collective dynamics in complex environments, which are relevant to synthetic as well as biological systems.