When in equilibrium, thermal forces agitate molecules, which then diffuse, collide and bind to form materials. However, the space of accessible structures in which micron-scale particles can be organized by thermal forces is limited, owing to the slow dynamics and metastable states. Active agents in a passive fluid generate forces and flows, forming a bath with active fluctuations. Two unanswered questions are whether those active agents can drive the assembly of passive components into unconventional states and which material properties they will exhibit. Here we show that passive, sticky beads immersed in a bath of swimming Escherichia coli bacteria aggregate into unconventional clusters and gels that are controlled by the activity of the bath. We observe a slow but persistent rotation of the aggregates that originates in the chirality of the E. coli flagella and directs aggregation into structures that are not accessible thermally. We elucidate the aggregation mechanism with a numerical model of spinning, sticky beads and reproduce quantitatively the experimental results. We show that internal activity controls the phase diagram and the structure of the aggregates. Overall, our results highlight the promising role of active baths in designing the structural and mechanical properties of materials with unconventional phases.
D.G. and J.P. thank E. Krasnopeeva, C. Guet, G. Guessous and T. Hwa for providing the E. coli strains. This material is based upon work supported by the US Department of Energy under award DE-SC0019769. I.P. acknowledges funding by the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie Grant Agreement No. 101034413. A.Š. acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant No. 802960). M.C.U. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie Grant Agreement No. 754411.
Grober D, Palaia I, Ucar MC, Hannezo EB, Šarić A, Palacci JA. Unconventional colloidal aggregation in chiral bacterial baths. Nature Physics. 2023. doi:10.1038/s41567-023-02136-x
Grober, D., Palaia, I., Ucar, M. C., Hannezo, E. B., Šarić, A., & Palacci, J. A. (2023). Unconventional colloidal aggregation in chiral bacterial baths. Nature Physics. Springer Nature. https://doi.org/10.1038/s41567-023-02136-x
Grober, Daniel, Ivan Palaia, Mehmet C Ucar, Edouard B Hannezo, Anđela Šarić, and Jérémie A Palacci. “Unconventional Colloidal Aggregation in Chiral Bacterial Baths.” Nature Physics. Springer Nature, 2023. https://doi.org/10.1038/s41567-023-02136-x.
D. Grober, I. Palaia, M. C. Ucar, E. B. Hannezo, A. Šarić, and J. A. Palacci, “Unconventional colloidal aggregation in chiral bacterial baths,” Nature Physics. Springer Nature, 2023.
Grober D, Palaia I, Ucar MC, Hannezo EB, Šarić A, Palacci JA. 2023. Unconventional colloidal aggregation in chiral bacterial baths. Nature Physics.
Grober, Daniel, et al. “Unconventional Colloidal Aggregation in Chiral Bacterial Baths.” Nature Physics, Springer Nature, 2023, doi:10.1038/s41567-023-02136-x.
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