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Optical Ratchets

Rather than simply measuring colloidal diffusion, optical tweezers have been used to exert a subtle influence on a particle's thermal wanderings [27]. Faucheux et al. moved an optical tweezer in a tight circular path while synchronously blinking it on and off. From the point of view of a particle, the time-averaged influence of the tweezer resembled a spatial sawtooth potential modulated in time by a square wave. When the potential was on, the particle remained near the minimum of one sawtooth. When it was off, the particle was free to diffuse over the crest to the next period of the sawtooth. If the particle did so, it would drift to the minimum of the next well in the next cycle. Otherwise, it would remain where it was. Over time, a small excess of particles was driven backwards along the sawtooth. This variant of Maxwell's Demon provided one of the first practical demonstrations of directed diffusion, the thermally driven process believed responsible for directed motion in a wide range of biological molecular motors including actin/myocin and kinesin/tubulin.

David G. Grier
Mon Feb 17 21:35:47 CST 1997