(63e) Hydrodynamically Reduced Microviscosity in Active Suspensions

Active colloidal suspensions containing motile agents exhibit non-equilibrium behavior and unusual rheological properties not seen in passive colloidal suspensions. We present a theoretical study on the microrheology of active suspensions, measured through the drag force that a microscopic probe experiences when forced to move at a constant velocity through a suspension of Active Brownian Particles. We show that when hydrodynamic interactions are accounted for using Active Stokesian Dynamics, the probe-measured viscosity of highly active suspensions can be lower than that of the solvent, and can even reduce to zero in some cases when active particles persistently swim faster than the speed of the probe. Analysis of hydrodynamic interactions between the probe and the active particles reveals that this is a direct result of the reduced relative mobility due to lubrication effects near the point of contact, which leads to an accumulation of active particles at the backside of the probe relative to its direction of motion, while vorticity aligns the active particles to swim in the direction of probe motion. As a result, the active particles are observed to help push the probe forward, reducing the apparent drag on the probe. The result can be generalized to swimmers described by the squirmer model.