How does the swimming motion of bacteria affect the mixing of passive scalars in chaotic flows? Answers to this question can lead to a better understanding of the formation of algal blooms in oceans and lakes, as well as potentially useful applications in vaccine and biofuel production. Our experiments show that the presence of swimming Escherichia coli in two-dimensional chaotic flows can hinder large-scale transport and reduce flow chaoticity. Locally, however, bacteria can significantly enhance small-scale mixing. Our simulations also reveal the potential mechanism for these phenomena being the transient accumulation and dispersion of bacteria near regions of large fluid stretching.

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