Brownian motion can be harnessed to create microscopic motors that are driven by molecular battering (for a recent example of an experimentally realized Brownian motor see Phys. Rev. Lett. 96, 190602 (2006), http://link.aps.org/abstract/PRL/v96/e190602). Now researchers at Hasselt University in Belgium and the University of Alabama in Birmingham have theorised that Brownian motion might be useful in designing molecular-scale refrigerators. Potentially, a Brownian refrigerator could help cool nanoscale machines or control the heat flow in molecular biology experiments.

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. Unlike heat pumps that move heat by compressing and expanding large volumes of fluids, a Brownian refrigerator controls heat flow heat by striking molecules to speed them up or slow them down.

The researchers propose a theoretical model of their refrigerator that consists of a rod piercing an insulating membrane. One end of the rod has an arrangement of flat paddles, much like those on a paddle wheel boat. On the other end the paddles are replaced with wedge shapes. Left to itself, the structure will spin, provided the molecules surrounding the wedges are warmer than the molecules surrounding the paddles, and heat will be moved through to the cooler side in the process of running the motor. If the motor could be forced to run backward it would move heat in the other direction—from the cool side to the hot side—forming the smallest possible refrigerator. The authors do not explicitly address the source of the force turning the Brownian refrigerator’s rotor, presumably leaving that challenge to future experimentalists.

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