Attach self-propelling bacteria to a cog and they'll set it spinning for you, say Italian physicists.
Last year, we looked at an idea for a bacteria-powered motor dreamt up by Luca Angelani and pals from the University of Rome in Italy. Their idea was to place a cog with asymmetric teeth into a bath of moving bacteria and wait for them to start it spinning for you, like carthorses pushing a millstone.
We said at the time that the idea sounds a bit like extracting kinetic energy from the random motion of particles, which is impossible because the motion is symmetric in time.
But Angelani and co say there is in important difference between Brownian 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.