Dynamics of ellipsoidal tracers in swimming algal suspensions

Abstract

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.

Enhanced velocity fluctuations in interacting swimmer suspensions

“…Pair-correlations are also crucially dependent on the swimming mechanism (pusher vis-á-vis puller), and lead to larger velocity variance and tracer diffusivities in suspensions of pushers (see figures 6 and 12). Thus, in contrast to recent work emphasizing the role of anisotropic tracers (Peng et al 2016;Yang et al 2016), spherical tracers already discriminate between pusher and puller suspensions on account of hydrodynamically induced correlations.…”

Diffusion of ellipsoids in laboratory two-dimensional turbulent flow

“…This anomalous coupling and diffusion was attributed to the mechanical properties of swimming microorganisms, being pushers or pullers, and to the corresponding strain generated by the active fluid. 37 We note that dense suspensions of bacteria have a tendency to produce swarming motion. [13][14][15][16][17] This swarming motion of bacteria seems to share some similarities with the behavior of coherent bundles of fluid particles recently reported in laboratory turbulent 2D flows.…”

Dynamic Coupling at Low Reynolds Number

“…Yang et al. validated this prediction using suspensions of the algae C. reinhardtii , and found that the long‐time particle diffusion along the major axis of the ellipsoids became more strongly enhanced as the concentration of the algal cells increased (Figure d) . It was therefore shown that, when the rotational degree of freedom was considered, the swimming mechanism of the active particles directly affected the coupling between the translation and rotation of asymmetric passive tracers.…”