The diffusion of a small tracer in a surrounding medium provides a reliable means for probing material properties of complex fluids. Particularly, such a method has been used to investigate unique features of active fluids—a novel class of non-equilibrium soft materials where each individual in the system propels itself. The behavior of spherical tracers in active fluids is most clearly illustrated by the diffusion of colloidal spheres in suspensions of swimming microorganisms, which exhibit a super-diffusive behavior at short times and a dramatically enhanced translational diffusion at long times. The enhanced diffusion of passive particles such as nutrient granules, and extracellular products is of great biological importance, which maintains an active ecological balance, stimulates biomixing, and promotes intercellular signaling and metabolite transports. However, few natural particles have the perfect spherical symmetry and usually possess more than translational degrees of freedom. It is still an open question how and to what extent the enhanced translational diffusion of an anisotropic particle is influenced by other degrees of freedom, especially by its rotation.
Recently, the research group led by Prof. X. L. Xu at Beijing Computational Science Research Center, together with Prof. X. Cheng’s group at the University of Minnesota investigated the diffusion of isolated ellipsoids in a quasi-two-dimensional bacterial bath. Their study shows a nonlinear enhancement of both translational and rotational diffusions of ellipsoids. More importantly, they uncover an anomalous coupling between particles’ translation and rotation that is strictly prohibited in Brownian diffusion. The coupling reveals a counter-intuitive anisotropic particle diffusion, where an ellipsoid diffuses faster along its minor axis. Combining experiments with theoretical modeling, it is shown that such an anomalous diffusive behavior arises from the generic straining flow of swimming bacteria. This work illustrates an unexpected feature of active fluids and deepens the understanding of transport processes in microbiological systems.
Fig. 1: Diffusion of an ellipsoid in quasi-two-dimensional bacterial bath. (A) Velocity field of swarming bacteria around an ellipsoid obtained from particle image velocimetry (PIV). Scale bar: 20 um. (B) Trajectory of an ellipsoid in a time interval Δt = 0.2 s. The trajectory is obtained from a custom algorithm for particle tracking velocimetry (PTV). The center of mass of the tracer at each time is indicated by one black dot. The color indicates the orientation of the tracer with respect to the x axis fixed in the lab frame.
Ref.: Y. Peng, L. Lai, Y. Tai, K. Zhang, X. L. Xu*, and X. Cheng*, Diffusion of Ellipsoids in Bacterial Suspensions. Phys. Rev. Lett. 116, 068303 (2016).