When placed in water, their long flagella—tails that propel them forward—create a so-called active bath. This dynamic environment helps form gel-like aggregates by acting like a small fire and raising the "temperature" to an equivalent of 2,000°C, similar to one a blacksmith needs to craft metals. It even manages to spin tiny micro disks.
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The scientists, therefore, needed to take a step back and come up with an experiment to clarify what was happening. To do so, Grober used a 3D nanoprinter to create smooth, symmetrical micro disks similar to hockey pucks. After introducing these "pucks" into the active baths filled with E. coli, they were surprised to see them spin clockwise, which negated the earlier hypothesis that symmetrical shapes do not turn.
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"It is a well-known result in our field that the counter-rotation of the body and flagella (tail) of an E. coli cause it to swim in clockwise circles near a solid surface," Grober explains.
"We realized that we could flip these dynamics upside down by confining the E. coli in a microscopic channel beneath the puck. These experiments utilize the exact same hydrodynamic effect to create, essentially, a microscopic and contactless engine, which drives the persistent rotation of the puck."