Forced microrheology of active colloids

Particle-tracking microrheology of dilute active (self-propelled) colloidal suspensions is studied by considering the external force required to maintain steady motion an immersed constant-velocity probe. If probe speed zero, suspension microstructure isotropic but exhibits a boundary accumulation bath particles at contact due their self-propulsion. As moves through suspension, distorted from nonequilibrium state, which allows us define microviscosity for using Stokes drag law. For slow probe, we show that exhibit swim-thinning behavior in gradually lowered passive as swim increases. When fast, activity obscured rapid advection and measured indistinguishable suspensions. Generally finite activity, velocity-thinning -- though with zero-velocity plateau lower than function speed. These behaviors originate interplay between hard-sphere excluded-volume interaction particle.