Microrheology of an active suspension of swimming bacteria

Optical tweezers allow the quantitative study of active biological suspensions, e.g., bacterial colonies, enzyme baths, and microswimmers. These active suspensions have the potential to do productive work on embedded objects and their environment. Here, we study the nonlinear response and force fluctuations of a probe particle suspended in a moderately dense suspension of swimming E.coli (0.2 vol frac). We study three physical processes: (1) the force fluctuations transferred to the probe particle, (2) the friction on the probe particle at varying Pe (a measure of persistent versus random motion), (3) the force relaxation as the particle returns to its equilibrium position. We find that at Pe<<1, the active bath experiences shear-thinning approaching the solvent viscosity but not lower. Between Pe 0.85 and 5.1, the active bath shear thickens, and at Pe > 8.5, the effective viscosity plateaus. These results are supported by recent theoretical predictions of the nonlinear rheology of an isotropic bath, extending experimental evidence to moderate densities. Our results set the stage for understanding the basic properties of force and energy transfer from active suspensions to embedded probe particles.