How bacterial colonies could drive rotors


E coli bacteria use motors called flagella to generate a force that pushes them along at a rate of up to 10 body lengths per second. That’s a fair rate of knots and in recent years several groups have used this force to turn microrotors. Their approach is to bond the bacteria to a rotor like carthorses to a millstone. That certainly works, but it’s time consuming and fiddly, especially when the workforce dies on you.

But there is a cleverer way, say Luca Angelani and pals from the University of Rome in Italy. Simply place an asymmetric cog in a bath of moving bacteria and they will start it spinning for you.

That sounds a bit like extracting kinetic energy from the random motion of particles, which we know to be impossible because the motion is symmetric in time.

But Angelani and co say there is in important difference between this and bacterial motion: the former is in equilibrium but the latter is an open system with a net income of energy provided by nutrients. This breaks the time symmetry allowing energy to be extracted in the form of directed motion.

Angelani and co calculate that their bacterial bath could turn an asymmetric gear at a rate of a few rpm, which is an interesting result.

Our findings can open the way to new and fascinating applications in the field of hybrid bio-microdevices engineering, and also provide new insight in the more fundamental aspects of nonequilibrium dynamics of active matter.

That sounds exciting and the effect doesn’t look hard to confirm experimentally. Which begs the question: what are they waiting for?

Ref: Self-Starting Micromotors in a Bacterial Bath

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