(526a) Bacterial Barriers in Chaotic Flows: A Lagrangian Description

How does the swimming motion of bacteria affect the mixing of passive scalars in chaotic flows? Answers to this question can lead to a better understanding of the formation of algal blooms in oceans and lakes, as well as potentially useful applications in vaccine and biofuel production. In this talk, I will show that the coupling between flow structure and swimmer motion can lead unexpected phenomena including bacterial trapping, adverse large-scale scalar transport, and manifold trafficking. In particular, experiments and simulations show that the flow Lagrangian Coherent Structures (LCS) are intimately connected to the the fate of bacteria and scalar mixing in time-periodic flows (see Figure). For example, it has recently found that presence of bacteria hinders passive scalar large-scale transport and reduces overall mixing rate in a 2D chaotic flow. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching, or unstable manifolds. On the other hand, at small scales, activity seems to enhance local mixing. Overall, our results show that once can design flows with specific structures and dynamical features to control the trapping and dispersion of bacteria, as well as enhance scalar mixing.