Mixing With E. Coli

Dye on the right half of this image shows a complex mixing pattern. On the left half, where dye was not introduced, concentrations of bacteria are highest in the areas with strong mixing.

What happens when a flow meets swimming micro-organisms? Does the flow affect the swimmers? And how do the swimmers affect the flow in turn? Those are the questions behind the experiment seen here. The apparatus contains a thin layer of saline water with swimming E. coli. Electromagnetism is used to mix the fluid in an array-like pattern that triggers chaotic mixing. To visualize what’s going on, dye is introduced into the right half of the image, while the left half remains undyed.

On the right side of the image, bright blue and white mark areas of high dye concentration, where strong mixing occurs. On the undyed left side of the image, pale blue streaks mark areas where E. coli are clustered. By comparing the two, we see that the micro-swimmers are clustered in the very same regions of flow marked by strong mixing. This result suggests strong interactions and the potential for feedback between the mixing flow and the swimmers. (Image and research credit: R. Ran et al.; see also 1 and 2)

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