  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 et al., Phys. Rev. Lett. 116, 068303
(2016)], 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.
