Van den Broeck and Kawai recently made a microscopic motor consisting of a single chiral, or asymmetrical, molecule. When placed between two reservoirs at different temperatures, this motor automatically moves in one direction to "rectify" the thermal fluctuations. In this way, it transfers heat from the high-temperature reservoir to the low-temperature one.

In their latest work, the researchers propose using an external force to drive the Brownian motor in the opposite direction so that it does the reverse -- that is, cause heat to flow from the colder region to the warmer one and so act as a refrigerator. This is much the same way that a household heat pump cools a room.

The researchers' theoretical model of the new fridge makes use of a chiral rod -- which has flat paddles (like those on a paddle-wheel boat) at one end and wedge-shaped paddles at the other -- piercing an insulating membrane. If the molecules surrounding the wedges have more kinetic energy than those surrounding the paddles the rod will spin, thereby moving heat from the warm side of the device to the cooler side. If a force is then applied to the rod, the motor runs "backwards" and moves heat in the opposite direction.

Such a fridge could, for example, be used to cool down semiconductor chips, channelling energy away from the centre of a chip to a cooling port by applying a torque to the molecules. It could also be used to cool down nanoscale machines. "Advances in nanotechnology will eventually bring machine sizes down to the limit where thermal fluctuations dominate," states Kawai. "Our Brownian machine magically exploits this random motion of molecules rather than fighting against it."