Hydrodynamics Reduce Microviscosity in an Active Suspension

Active matter generates self-propulsion by consuming external energy, resulting in non-equilibrium behaviors not seen in passive colloidal suspensions. We present a theoretical study on the microrheology of active Brownian suspensions, modeled with a microscopic probe moving through an active suspension at constant speed, while accounting for full hydrodynamic interactions using Active Stokesian Dynamics. The apparent viscosity experienced by the probe becomes lower than the solvent viscosity when the swimming activity is high compared to thermal Brownian motion. Boundary layer analysis reveals that this is a direct result of hydrodynamics reducing the relative mobility between the probe and the active particles, while vorticity aligns the swimming direction of active particles with the direction of probe motion. The result is generalizable to swimmers described by the squirmer model.