Water that walks

Ten years ago, Manoj K. Chaudhury, the Dow Corning associate professor of chemical engineering at Lehigh University, discovered method to make droplets of water take a stroll up a slope.

Chaudhury said he had coaxed a microlitre of water to ‘run uphill’ on a surface of polished silicon at about 1 mm per second by varying the degree of hydrophobicity on the surface.

The change in surface properties, said Chaudhury, created an imbalance of surface tension forces, or a gradient of low to high interfacial energy, helping to propel the water upward on a tilted horizontal plane.

Today, the creeping droplets that defied gravity a millimetre at a time are said to have acquired the speed of an Olympic sprinter.

Chaudhury said that by passing saturated steam over a hydrophobic surface possessing a surface tension gradient, droplets of water can be induced to move at rates of centimetres, even a meter or more, per second.

‘This phenomenon results from the combination of the surface gradient with the fast condensation,’ said Chaudhury.

Chaudhury first presented his ideas on surface tension gradients at Lehigh at a chemical engineering department seminar in 1994.

In attendance was John C Chen, the current dean of the PC Rossin College of Engineering and Applied Science.

Chen suggested that Chaudhury apply the gradient effect to heat-transfer problems.

Previously, Chaudhury could make the water drops move in only one way: from point to point. The increased energy created by the condensation is now said to make it possible to make the drops move radially, or out from the centre of a surface, as well as up and down parallel channels, or columns.

Chaudhury said the new phenomenon can be potentially applied to heat transfer problems, especially those involving systems operating in zero or micro-gravity.

In these systems, he said, a surface tension gradient, performing the function that gravity, could pump water radially from a horizontal surface, preventing liquid build-up and thus improving the efficiency of heat transfer.

Another potential application, he said, is to microfluidic devices, especially the microchips equipped with microfabricated miniature fuel cells.

‘A chemical reactor on a microchip would run on a very small amount of liquid,’ said Chaudhury. ‘That liquid would have to be pumped from one end of the reactor to another. It would be very difficult to use external sources to do this, but surface energy could act as a pump.’

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