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Water will walk uphill under its own steam
[January 21, 2006]

Water will walk uphill under its own steam

(New Scientist Via Thomson Dialog NewsEdge)LIQUID droplets have been made to "walk" across horizontal surfaces and even uphill quite literally under their own steam. Such self-propelled liquids could be used to cool microprocessors.

To get droplets of water and other liquids to do this, Heiner Linke and his colleagues from the University of Oregon in Eugene and the University of New South Wales in Sydney, Australia, used a brass surface etched with ratchet-like corrugations at a temperature considerably hotter than the boiling point of the liquid .

Because the surface is so hot, 220 C for water droplets, for instance a layer of vapour known as a Leidenfrost layer forms between the surface and the droplet. Suspended on this cushion, the droplet moves easily under the slightest horizontal force and, on the uneven surface, appears to move on its own. "We have observed droplets zooming along at up to 5 centimetres a second," Linke says.


Linke and his colleagues are not yet certain where the horizontal force comes from, but they think that the vapour which is being continually generated between both the liquid and the hot surface escapes from under the droplet and then generates a small propulsive force. "The asymmetry of the ratchet-like surface causes the vapour under the droplet to flow mostly in one direction," says Linke. "And, like a boat on a river, the droplet is dragged along on the vapour flow."

So far, the team has observed the phenomenon in all the liquids they have investigated, including liquid nitrogen, methanol, ethanol and water, with boiling points ranging from ?196 C to 151 C. Typically, the droplets are a millimetre across and the corrugations are 0.1 mm high.

The droplets can accelerate by 1 to 2 metres per second per second. "We have observed the droplets travelling up to a metre, until they eventually evaporate," says Linke. The team also found that the droplets could climb up inclines of up to 10 degrees (www.arxiv.org/physics/0512246
).

The self-propelled liquids could be used in pumps powered by waste heat from microprocessors, for example. "It would be necessary only to have the liquid in a channel with ratchet-shaped inner walls," he says. "There would be no moving parts and no need for external power." The liquid would carry away the heat, cooling the microprocessor.

While the group has filed for a preliminary patent on such a cooling system, they don't yet know if the system will work at smaller scales. "I am pretty confident that there might be niche applications," says Linke. "But it is too early to say whether the idea might beat [other] competing concepts for microprocessor cooling."

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