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Harnessing Brownian Motion

October 1, 1997

Small particles suspended in a liquid are constantly buffeted by collisions with other molecules, causing them to jiggle erratically in a manner known as Brownian motion. The phenomenon-first observed by botanist Robert Brown in 1824 and later described theoretically by Albert Einstein-is ubiquitous in nature, an inevitable consequence of thermal energy in the environment. Now, University of Chicago biochemist R. Dean Astumian claims that by applying external forces and employing various tricks, we can “bias” these otherwise random meanderings and “get things to move more one way than the other,” thus laying the groundwork for potentially useful devices.

One arrangement would consist of a tilted pipe that features an array of positive and negative electrodes attached along one side and that also contains a fluid filled with negatively charged particles. When the electricity is turned on, electrical attraction will induce all the particles to accumulate at the positive electrodes. When the electricity is turned off, gravity will cause most particles to drift downhill. But owing to Brownian motion, some occasionally will go uphill. If these upward-traveling particles make it past the negative electrode, which is located just barely above the positive, they will be forced uphill to the next positive electrode when the electricity is turned back on. The rest of the particles that drifted downhill will return to the positive electrode where they were originally congregated. “By repeating this process, turning the electricity on and off at precisely the right times, we can generate net uphill motion,” Astumian explains.

If bigger particles-composed of the same material and bearing the same negative charge-are added to the pipe, they will tend to move downhill more than the smaller particles, because they experience a stronger tug from gravity with comparatively less frictional drag. “If you do this right, you can end up with a situation where the big particles move downhill while the small particles move uphill,” Astumian says. That could be extremely useful, he adds, because at present, “there are no really good ways of separating particles by size.” The “biased Brownian motion” approach, he says, might provide the basis for practical devices that could continuously separate proteins or other molecules in biological or chemical processing.

But even if such applications don’t pan out, the research should, at the very least, contribute to a new appreciation of the random, thermal-driven motion also known as thermal noise. “We’ve always looked at thermal noise as something to get rid of,” Astumian says. “But now we’re taking the opposite approach and trying to use it constructively.”

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