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Heiner Linke and colleagues from the universities of Oregon and New South Wales have succeeded in getting liquid droplets to move across horizontal surfaces and travel uphill.

The phenomenon was achieved by heating an etched brass surface so that it is considerably hotter than the liquid's boiling point. For example, when using water they heated the brass to between 200° and 300°.

The high temperatures cause a Leidenfrost layer of vapour to form between the liquid and the surface. The liquid then moves across this layer apparently under its own steam, as it were.

'We have observed droplets zooming along at up to 5cm a second,'

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Linke said.

He added that the researchers are not yet sure what causes the liquid to move, although they think that as the vapour forms it 'escapes' from beneath the liquid creating small force.

'The asymmetry of the ratchet-like surface causes the vapour under the droplet to flow mostly in one direction,' Linke explained. 'And, like a boat on a river, the droplet is dragged along on the vapour flow.'

The team has found that the phenomenon existed for all the 1mm-across liquid droplets that they have so far tested, including liquid nitrogen, methanol, ethanol and water. The droplets travel as far as one metre before evaporating and can move up surfaces inclined by as much as 10°.

Hinke hypothesised that 'self-propelled' liquids could be used to cool microprocessors, using heat from the chips to power pumps that would feed the liquid into channels on the processor's inner walls. The liquid would then draw away excess heat.

To read Hinke' s paper and see QuickTime movies of the droplets in action go to tinyurl.com/bt22v.