Nonequilibrium fluctuations and nonlinear response of an active bath

Abstract

We analyze the dynamics of a passive colloidal probe immersed in an active bath using an optical trap to study three physical processes: (1) the nonequilibrium fluctuations transferred to the probe by the active bath, (2) the friction experienced by the probe as it is driven through the active bath, and (3) the force relaxation of the probe returning to its equilibrium position. We measure the local force dynamics where all of the following characteristics are of O(1): the size of the probe colloid relative to the active bath particle; the size of the probe colloid relative to the characteristic run-length of an active particle; and the timescale of probe movement to the persistence time of an active particle. We find at Péclet (Pe) ≪1 the active suspension exhibits shear thinning down to the solvent viscosity (but not below); at 0.85