Transport Barriers and Elliptic Islands—Mixing with Swimming Microorganisms in Chaotic Flows

We investigate the effects of swimming microorganisms on the transport and mixing of a tracer in two-dimensional chaotic flows. Experiments show that microorganisms lead to transport barriers through which the tracer flux is signifcantly reduced. These transport barriers coincide with vortex-like dynamical structures known as the elliptic Lagrangian coherent structures (elliptic LCSs). Simulations further show that elliptic LCSs repel elongated swimmers and lead to swimmer depletion within the Lagrangian vortices. A simple mechanism shows that such depletion is due to the preferential alignment of elongated swimmers with the elliptic LCSs.