Thursday, February 5, 2009

Letting bacteria work for you

In a fascinating paper just published in Physical Review Letters (January of this year), a small research group from the University of Rome describes their elaborate numerical simulations of micromotors (micron-scale gears, essentially) in a chaotic bacterial bath.
For some time, microscopic motors, driven by tiny mule-teams of bacteria or other self-propelling entities, have been considered an interesting and plausible way of converting chemical energy to useful mechanical energy. The PRL describes how such a mechanism might just be constructed.

Starting with relatively simple models for the bacteria themselves (oblong spheroids, self-propelled but subject to tumbling, Brownian motion, hydrodynamics, etc) and two different shapes for the "rotor" (one symmetric and one asymmetric), the forces and torques exerted on and by each of 1092 simulated bacteria are calculated, and the equations of motion for each numerically integrated using a Runge-Kutta method. Though many of the initial parameters were derived from a great deal of physics and biology, the process itself is very simple: press start and iteratively watch what happens.
And something incredible does happen - the rotor starts to turn (see figure 1 above; note the arrow). The asymmetric rotor "spins up" and then hovers around a non-zero, non-negligible angular momentum (see figure 2 below). The symmetric rotor, on the other hand, stays essentially stationary. It appears as though the asymmetry is the key to spontaneously "rectifying" the chaotic and random motion of the bacteria into the usable, directed motion of a rotor.
While this paper describes only a few numerical simulations (a forthcoming paper is planned to discuss the results of all the different input parameters), it is a tremendous step toward actually creating a method for deriving mechanical energy from something we typically try to avoid - bacteria. Billboards of the future will exclaim, in great blue letters, "Let bacteria work for you!" alongside photos of happy families with their home's lights blazing.

Ok, perhaps it's still a bit far off, but until then, I'm happy to read about some interesting and beneficial results that were spawned by a simple numerical code.

Luca Angelani, Roberto Di Leonardo, Giancarlo Ruocco (2009). Self-Starting Micromotors in a Bacterial Bath Physical Review Letters, 102 (4) DOI: 10.1103/PhysRevLett.102.048104

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