Unconventional colloidal aggregation in chiral bacterial baths

D Grober and I Palaia and MC U├žar and E Hannezo and A Saric and J Palacci, NATURE PHYSICS, 19, 1680-+ (2023).

DOI: 10.1038/s41567-023-02136-x

Colloidal aggregates are conventionally formed by particle aggregation under thermal fluctuation. Now the structure and mechanical properties of aggregates can be controlled by an active bath of swimming Escherichia coli. 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.

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