Enhanced diffusion of passive tracers immersed in active fluids is a universal feature of active fluids and has been extensively studied in recent years. Similar to microrheology for equilibrium complex fluids, the unusual enhanced particle dynamics reveal intrinsic properties of active fluids. Nevertheless, previous studies have shown that the translational dynamics of spherical tracers are qualitatively similar, independent of whether active particles are pushers or pullers - the two fundamental classes of active fluids. Is it possible to distinguish pushers from pullers by simply imaging the dynamics of passive tracers? Here, we investigated the diffusion of isolated ellipsoids in algal C. reinhardtii suspensions - a model for puller-type active fluids. In combination with our previous results on pusher-type E. coli suspensions [Peng, Phys. Rev. Lett. 116, 068303 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.068303], we showed that the dynamics of asymmetric tracers show a profound difference in pushers and pullers due to their rotational degree of freedom. Although the laboratory-frame translation and rotation of ellipsoids are enhanced in both pushers and pullers, similar to spherical tracers, the anisotropic diffusion in the body frame of ellipsoids shows opposite trends in the two classes of active fluids. An ellipsoid diffuses fastest along its major axis when immersed in pullers, whereas it diffuses slowest along the major axis in pushers. This striking difference can be qualitatively explained using a simple hydrodynamic model. In addition, our study on algal suspensions reveals that the influence of the near-field advection of algal swimming flows on the translation and rotation of ellipsoids shows different ranges and strengths. Our work provides not only new insights into universal organizing principles of active fluids, but also a convenient tool for detecting the class of active particles.
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We acknowledge P. Lefebvre for providing us with the C. reinhardtii strain and helping us with algae culturing. We also thank B. Zhang, L. Gordillo and D. Samanta for help with experiments and L. Lai for fruitful discussions on theory. The research was supported by ACS Petroleum Research Fund (54168-DNI9) and by the David & Lucile Packard Foundation. C.T. thanks support from the Coating Process Fundamentals Program (CPFP) at University of Minnesota. X.X. acknowledges support by the National Natural Science Foundation of China, Grant No. 11575020.
© 2016 American Physical Society.