Nonequilibrium fluctuations and nonlinear response of an active bath

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 Peclet (Pe) << 1 the active suspension exhibits shear thinning down to the solvent viscosity (but not below); at 0.85 < Pe <= 5.1, the active bath shear thickens; and at Pe >= 8.5, the effective viscosity of the active bath shows a decreased effect of thickening and plateaus. These results are in agreement with recent modeling and simulations of the nonlinear rheology of an isotropic active bath, providing experimental verification, and suggesting the model predictions extends to moderately dense suspensions. Further, we observe that the distribution of force fluctuations depends on Pe, unlike in passive equilibrium baths. Lastly, we measure the energy transfer rate from the active bath to the probe to be (J) approximate to 10(3) k(B)T/s, which leads to an increase in the effective diffusion of the probe by a factor of similar to 2.

Automated summary

In this study, we investigated the dynamics of a passive colloidal probe in an active bath using an optical trap. The results revealed that the viscosity of the active bath exhibits shear thinning, shear thickening, and plateau effects depending on the value of the Peclet number. Additionally, we observed that the distribution of force fluctuations in the active bath is different from that in passive equilibrium baths. Moreover, the energy transfer rate from the active bath to the probe was measured.