Abstract
Active fluid, such as bacterial suspension and self-propelled colloids, performs intriguing features other than the complex fluids in equilibrium, including collective motion, abnormal viscosity, giant number fluctuations and enhanced diffusion. Here, we studied the diffusion of an ellipsoid in a free-standing film of E. coli. Using high-speed digital video microscopy, we imaged the motion of the ellipsoid and found that both the translational and rotational diffusion of the particles weredramatically enhanced by the micro-swimmers. The enhancement varied with the concentration of micro-swimmers: diffusion was linearly enhanced at low concentrations whereas the enhancement became non-linear at high concentrations because of the collective motion of bacteria. Moreover, we studied the coupling between the translational and rotational motions, which resulted from the anisotropic diffusion in the body frame. At high bacterial concentrations, the ellipsoid exhibited anomalous coupling behavior which was due to larger diffusion coefficient along the minor axis of an ellipsoid. This counter-intuitive diffusion behavior was well explained by considering the dipolar flow field of microswimmers. This anomalous behavior is expected to be universal in active systems.