(358j) Diffusion of Ellipsoids in Active Fluids

Enhanced diffusion of passive tracers immersed in active fluids is a universal feature of active fluids and has been studied extensively in recent years. In analogy to microrheology used in equilibrium complex fluids, it is reasonable to assume that the unusual dynamics of passive tracers reveal intrinsic properties of active fluids. Nevertheless, experiments, simulations and theories have all shown that the translational dynamics of spherical tracers in active fluids are qualitatively similar, independent of whether the active fluids are pushers or pullersâ??the two fundamental classes of active fluids. Is it possible to distinguish between pushers and pullers by simply imaging the dynamics of passive tracers? Here, we experimentally studied the diffusion of isolated ellipsoids in both pusher and puller active fluids using suspensions of E. coli and algae as our model systems. Due to the asymmetric nature, ellipsoids possess a rotational degree of freedom, which has not been studied in the context of active fluids. Surprisingly, we found that although the translational and rotational diffusions of ellipsoids show the same enhancement in both pushers and pullers similar to that of spherical tracers, the coupling between the two degrees of freedom exhibits completely opposite trends in the two classes of active fluids. We showed that these opposite translation-rotation couplings arise from a sharp difference in the anisotropy of the body-frame diffusion of ellipsoids. An ellipsoid diffuses fastest along its major axis when immersed in pullers, whereas it diffuses slowest along the major axis in pushers. Based on these experimental findings, we have developed a simple mean-field hydrodynamic model, which qualitatively explains the observed behaviors. Our study provides not only a new insight into organizing principles of active fluids, but also a convenient tool for probing the fundamental classes of active fluids. We acknowledge support from ACS PRF (54168-DNI9) and the Packard Foundation.