2023-09-13 19:00:18
Observation of aggregations of small beads in a fluid under the effect of the natural agitation of bacteria. On the left, during an experiment. On the right, calculated by numerical simulations reproducing the phenomenon. IVAN PALAIA
To sculpt the material, the potter places his fingers in the clay. The blacksmith heats, quenches and strikes the steel. The cook emulsifies or whips his mixtures. And the physicists? They can now do all this at once, on microscopic scales, thanks to the famous bacteria of biologists, Escherichia coli (E. coli), and is 2 micrometers long.
In Nature Physics of July 27, an Austrian team gives the details of this original recipe which made it possible to create ” materials “ unknown until now. Into a liquid, they throw a handful of 2.2 micrometer wide beads, capable of sticking together if they get too close. The agitation of the fluid, greater or lesser depending on the heating, helps in this rapprochement and leads the individual objects to aggregate into small packages. So far, nothing new.
However, if we turn off the heat and throw in a pinch of bacteria, another film unfolds. The beads also aggregate, but more quickly, into larger and more compact shapes. “I’m not sure we could make these objects with other techniques”underlines Jeremy Palacciprofessor at the Austrian Institute of Science and Technology in Klosterneuburg, near Vienna, and leader of the team. “Each bacteria acts like a little spoon that mixes on micrometric scales, he poeticizes. So it’s not like we have warmer water, which agitates the particles more. »
Furthermore, as a surprise to the experiment, these shapes rotate on themselves because of the rotation of the bacteria, always in the same direction. A clever experiment, not at the bottom of a tank but in bubbles, made it possible to link the rotation of the propelling caudal flagella ofE. coli to the macroscopic movement of aggregates. More precisely, the latter are in fact gels, that is to say a united architecture, in which pulling on one end causes all the rest by elasticity – and not by rigidity as in a solid.
“Green” manufacturing
“This work belongs to the field of active soft matter. It shows how, thanks to this active material, we can obtain new types of materials”appreciates Zvonimir Dogic, from the University of California, Santa Barbara. In 2021, he added molecular motors to fluids to form more or less deformed drops in an emulsion and break them up.
For the moment, the Austrians have not been able to “shoot” their gel to verify that it would have original properties. They would actually have to make these materials in three dimensions whereas, for the moment, the balls fall to the bottom of the container and form more mats than gelled sponges.
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