Mixing & transport of microorganisms in 2D time-periodic flows

In this talk, we examine the effect of swimming bacteria on transport, diffusion, and mixing in time-periodic flows. An oscillatory two-dimensional flow is generated by driving a sinusoidal current through a conducting fluid (salt water) that is situated atop an array of magnets. We perform experiments with an ordered lattice to create regular vortices and with a random configuration to create a spatially disordered flow pattern; the Reynolds number ranges from 0.1 to 100. Two types of fluids are used: (1) a simple Newtonian liquid (water) and (2) a mixture of water and bacteria (Vibrio cholerae). Velocimetry data are used to calculate the flow stretching fields and Lyapunov exponents. Results with plain salt water are compared to results with the addition of varying concentrations of V. cholerae. We find that the addition of active bacteria, even in dilute quantities, results in significant changes to the stretching fields even though the Eulerian velocity fields remain quite similar. We also perform experiments with fluorescent dye, both with and without bacteria, to more directly characterize mixing. These data also show a substantial difference between the active suspension and the control case, even with small concentrations of bacteria.