The world's smallest refrigerator

Two theoretical physicists have used the Brownian motion to design a molecular-scale refrigerator powered by 'nano' paddles. And they think that this Brownian refrigerator will one day help cool nanoscale machines.

According to Nature, two theoretical physicists have used the Brownian motion to design a molecular-scale refrigerator powered by 'nano' paddles. And they think that this Brownian refrigerator will one day help cool nanoscale machines. Their mini fridge is composed of two pools of molecules separated by an insulating membrane. A rod crosses this membrane, with wheels of flat paddles on one side and wedge-shaped paddles on the other. By forcing a set of paddles to run counter-clockwise, one of the pool gets warmer and the other one cooler. Read more...

Here is an introduction from the Nature article.

The proposal comes from a pair of theoretical physicists, who admit that it would take some spectacularly cunning molecular engineering to make their device a reality. But for now, Chris Van den Broeck from Hasselt University in Belgium and Ryoichi Kawai of the University of Alabama at Birmingham, say the odd trick in their idea will set thermodynamics researchers thinking.

Their idea is based on the Brownian motion which is "the random jitter of small particles that are bumped about by molecules," according to an American Physical Society news release.

On the image below, you can see a conceptual design for such a Brownian refrigerator. The left part describes the chiral -- or asymmetrical -- rotor placed across an insulating wall. [Note: an object or a system is called chiral if it differs from its mirror image, according to Wikipedia.] And the right part of the figure shows you how "a chiral biological molecule will channel heat across a membrane when a small external torque is applied." (Credit: C. Van den Broeck and R. Kawai)

Brownian Refrigerator

Differences in the temperatures of molecules in two regions lead to the heat flow that drives Brownian motors. Therefore, the researchers propose, using some external force to drive a Brownian motor in reverse could cause heat to flow from a colder region to a warmer one, much as household heat pumps cool homes with a motor that moves heat outside.

So far, this is only a theory.

The authors do not explicitly address the source of the force turning the Brownian refrigerator's rotor, presumably leaving that challenge to future experimentalists.

And Nature confirms that this kind of fridge is not ready yet -- even for use in the labs.

Van den Broeck suggests that lasers or magnets might be used, but he admits that's on the borderline of what is feasible. "I like theoreticians," laughs Anders Kastberg, a physicist at Umeå University, Sweden. "They don't have to bother about reality."

This research work is available today on arXiv under the name "Brownian refrigerator" and should soon be published in Physical Review Letters. Here are the links to the abstract and to the full paper (PDF format, 4 pages, 161 KB) from which the above image has been extracted.

Sources: Mark Peplow, Nature, May 30, 2006; American Physical Society news release, via EurekAlert!, May 23, 2006; and various web sites

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