Active mixing of swimming bacteria in laminar shear flows

We present experiments on the motion of smooth-swimming and tumbling bacillus subtilis bacteria in steady and time-periodic laminar flows. The flows are either a channel (Poiseuille) flow in a PDMS cell or a Kolmogorov flow (alternating jets) in an acrylic cell, forced magnetohydrodynamically. For both flows, passive mixing in the flow is ordered -- even if the flow is time-periodic -- since the motion parallel to and perpendicular to the jets are decoupled from each other. For self-propelled impurities in the flow, however, the swimming  direction introduces an additional phase space variable that allows for the possibility of chaotic trajectories. We track the motion of swimming microbes in these flows and (1) investigate conditions where the trajectories are chaotic or ordered; (2) measure variations in the microbe density for both smooth-swimming and tumbling microbes; and (3) study "flights" in the microbe orientations that are predicted by the structure of the (x,y,θ) phase space.