Recent Press Releases

Swimming in treacle

PR11(07)

Tue, 22 May 2007

If you are a sperm or a bacterium, swimming isn’t at all like it is for an Olympic swimmer. At these tiny scales of perhaps a thousandth of a millimetre, water seems very much ‘thicker’ – more viscous – than it does for a human-sized swimmer, so that the liquid settles into stillness almost at once after any movement. That might sound as though it makes swimming hard work, but in fact it means that entirely new kinds of swimming motion become possible: in effect, a swimmer can ‘climb’ through the water.

Chris Pooley of the University of Oxford will propose some designs for artificial microscopic swimmers that take advantage of this difference in a talk at the Condensed Matter and Materials Physics conference, organised by the Institute of Physics on 12 April.

Bacteria and sperm move around by shaking or rotating long, thin filaments called flagella. But recently, some researchers have proposed a different design for microscopic swimmers, in which small beads are linked in a chain by rods that can extend and contract. A specific sequence of such movements in a chain of three beads connected in a straight line can propel the structure through the ‘thick’ liquid. In 2005 a team of scientists in France and the USA made a string-of-beads propulsion unit from microscopic magnetic spheres, which they attached to a red blood cell and used to propel it in water. Devices like this might be useful in biomedicine or in the development of micro-robots for exploring watery environments.

Pooley has studied the way a three-bead swimmer works, and finds that by including a kink in the chain, it can be made to move not just in one direction but in two. The direction of motion, as the two linking arms expand and shrink, depends on the angle of the kink, and can be switched from backwards to forwards. This ‘bent’ swimmer can also rotate if the lengths of the two arms change to different degrees, altering the direction of motion.

Pooley then thought about what happens if more spheres are added to the chain. As this number increases, the swimmer gets more and more efficient at converting the energy of arm ‘strokes’ (length changes) into movement through the liquid. These results show how the behaviour of artificial microscopic swimming devices could be improved and made more versatile. He has also studied how groups of such swimmers might become spontaneously aligned with each other because of the effects of the flow patterns they set up in the liquid. This can produce the kind of ‘flocking’ motions seen in schools of fish.

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