Nanowerk July 27, 2023
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. Researchers in Austria showed 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. They observed a slow but persistent rotation of the aggregates that originated in the chirality of the E. coli flagella and directed aggregation into structures that were not accessible thermally. They reproduced and explained the experimental results quantitatively. They showed that internal activity controlled the phase diagram and the structure of the aggregates. According to the researchers their results highlight the promising role of active baths in designing the structural and mechanical properties of materials with unconventional phases… read more. Open Access TECHNICAL ARTICLE
Experimental set-up and bright-field images of aggregates. Credit: Nature Physics (2023), 27 July 2023Â