Design eases nano locomotion

At very small scales the effect of turbulence disappears, making liquids more difficult to travel through. The viscosity of water at small scales is more like honey than a quickly-flowing liquid.

This makes it difficult to design microscopic machines that can move through water. The trick is to design a cyclical motion that is not identical in the reverse direction. In the absence of turbulence, which allows for gliding, reciprocal motion simply holds a swimmer still.

Researchers from the Institute for Advanced Studies in Basic Sciences in Iran and the Max Plank Institute for Physics in Germany have designed a swimming machine that meets the challenge. The swimmer consists of three rigid spheres linked by narrow rods.

The design could eventually be used to propel machines whose size is in the molecular realm. These machines could be constructed from molecules that change length in the presence of light or from motor proteins used by microbes.

Key to the swimming ability is a nonreciprocal four-step periodic motion: the right arm stays fixed while the left arm shortens, the left arm stays fixed while the right arm shortens, the right arm stays fixed while the left arm lengthens, and finally the left arm stays fixed while the right arm lengthens.

The mathematics behind the motion can be worked out relatively easily, which makes it easier to use the motion in practical machines.

It will be one or two decades before the design can be used in practical devices, according to the researchers. The work appeared in the June 16, 2004 issue of Physical Review E.