We conducted experiments using a typical centimeter swimmer, A. salina, and an electromagnetically driven quasi-two-dimensional flow to study the interaction between active matter and flow. The flow spanned from time-independent cellular flow to weakly time-dependent regimes. We focused on the impact of swarming active matter on hyperbolic Lagrangian coherent structures (LCSs) that mark the most straining regions in the flow. There is one decade of scale separation between active matter agents and the length scale of LCSs. We illustrated that the impact of active matter on LCSs was much more significant compared to localized random noise with similar energy input. In addition, we revealed that the perturbation generated by active matter could couple with the background flow and further deform the LCSs. In addition to the impact on the most straining hyperbolic regions of flow, we also revealed that the rotational elliptical region of the flow was much more susceptible to active matter perturbation in a Lagrangian perspective. We further described how the influence of active matter changed with their number densities and background flow intensities. We revealed that the LCSs could be decently altered even at a small number density of active matter. Through this work, we aim to provide valuable insights and draw attention to the problem regarding the interaction between active matter and external flow structures.

• Received 8 November 2023
• Accepted 20 February 2024

DOI:https://doi.org/10.1103/PhysRevFluids.9.033101