Enhanced Transport Barriers with Swimming Microorganisms in Chaotic Flows

How does the swimming motion of microorganisms affect the transport and mixing of a passive scalar in complex flows? The answers to this question can potentially improve our understanding of the spread of pollutants (e.g., algal blooms, oil spills) in oceans and lakes, as well as stimulate the development of industrial applications (e.g., biofuel and vaccine productions). In our experiments, we investigate the chaotic mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli. Results show the interaction between the swimming microbes and the dynamical structures of the flow (i.e., Lagrangian coherent structures) can lead to enhanced transport barriers across which the fluxes of the passive scalar are reduced. Discreet particle simulations show that the Lagrangian transport barriers trap passive particles, but repel active particles and cause depletion zones. Overall, our results show that the coupling between activity and Lagrangian coherent structures has nontrivial effects on transport and mixing.