Forced microrheology of active colloids

TL;DR

This study investigates how active colloidal suspensions respond to external forces using particle-tracking microrheology, revealing swim-thinning and velocity-thinning behaviors influenced by particle activity and interactions.

Contribution

It introduces a microrheological framework to quantify the microviscosity of active suspensions and uncovers how activity alters their flow behavior compared to passive suspensions.

Findings

Active suspensions exhibit swim-thinning behavior with increasing swim speed.

At high probe speeds, activity effects are masked, resembling passive suspensions.

Velocity-thinning behavior with a lower zero-velocity plateau is observed due to activity.

Abstract

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