Law and Disorder: Chance fluctuations can rule the nanorealm

Whether it’s the gasoline-to-motion transformation of automobiles or the electricity-to-cooling action of refrigerators, all processes squander energy. They vent that waste in the form of heat. It’s a law of thermodynamics, and no one has ever witnessed a sustained violation of it.

On the minute scales of cells and molecules, however, brief reversals of the usual rules routinely occur. Tiny mechanisms run in reverse or draw their power from random, normally untappable thermal motion in the surroundings. Such small systems, on average, still obey thermodynamics laws, although some theorists predict that certain quantum structures may not (SN: 10/7/00, p. 234: Now, researchers in Australia report that they have experimentally confirmed a theory that enables them to predict how often and under what circumstances reversals will dominate the behavior of a classical tiny system.

The new observations could become a reality check on the burgeoning field of nanotechnology, the scientists say. Working in an unfamiliar realm, many nanodevice makers today can’t predict which of their mechanisms will actually work as planned.

Moreover, because the living machinery of cells and microorganisms also operates on the nanoscale, the Australian work could lead to new biological insights as well.

To track transient reversals of a thermodynamics law, Denis J. Evans of the Australian National University in Canberra and his colleagues manipulated latex beads about the size of red blood cells. They used an infrared laser as if it were an ultratiny tweezers.

Imagine pulling a toy submarine through calm water by a rope tied to its prow. Because the water provides drag, the boat will lag behind the puller and rope, that is, unless it gets some sort of push.

That’s roughly what happens to the latex beads. When Evans and his colleagues tugged their beads through water with their optical tweezers, sometimes a bead would slightly lead the laser, says Debra J. Searles of Griffith University, a member of the team. In such instances, the random motion of the water molecules was contributing to the bead’s forward motion.

In tests that spanned from one-hundredth of a second to 10 seconds, the scientists found that for periods up to almost 2 seconds, the thermodynamic reversals could dominate the bead-dragging runs. The results, scheduled to appear in the July 29 Physical Review Letters, confirm predictions of a theory about the effect of random fluctuations developed by Evans and Searles almost a decade ago.

Searles says the new findings will come as a surprise to most scientists because the prevailing wisdom has been that such reversals have a major impact only on much smaller scales of size and time. “It’s a tiny bead, but it’s still a lot of atoms,” she says.

Daniel P. Sheehan of the University of San Diego is not wowed by the size at which the effects appear. After all, he notes, ever since the 19th-century discovery of Brownian motion–the jiggling of pollen-grain-size particles in fluids because of random molecular bombardment–scientists have known that thermal motion can push fairly big particles around.

However, Sheehan was impressed by how long the thermodynamic reversals could dominate in the new tests. “It goes against my intuition that you could see [that effect] for as long as a tenth of a second,” he says.

The result suggests that random thermal fluctuations could become a proverbial monkey wrench for many nanomachines, Searles says. Instead of going forward, for example, they might sometimes go backward. Even so, she says, nanomachine makers may find the new work useful as a tool for predicting whether their plans may go awry.

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