Jumping Water Droplets Could Heighten Energy Efficiency

Thursday, October 24th, 2013
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Scientists have inadvertently discovered that water droplets leaping away from superhydrophobic surfaces harbour an electric charge, in a finding that could lead to more efficient power generation.

During an extension of prior research work, a team of scientists from MIT discovered that under certain conditions water condensation will literally leap away from the surface of superhydrophobic materials, instead of sliding down gradually as a result of gravity’s action.

The phenomenon occurs when at least two droplets come together, triggering a release of excess surface energy.

High-speed video showed that when droplets jump they repel each other mid-flight – a reaction which had not previously been observed. Subsequent experiments conducted by the MIT scientists using positive and negative electrodes found the effect is caused by a net positive electrical charge forming on the droplets when they leap from the surface.

MIT postdoc Nenad Miljkovic, co-author of a paper on the research team’s findings published in the journal Nature Communications, said the droplets become charged as they coalesce, creating a paired layer of positive and negative charges on their surfaces. Adjacent drops then react when they come into contact, leaping from the surface in a process which occurs “so fast that the charges separate,” with some of the charge retained by the departing droplets and the rest left on the surface.

The surprise finding could greatly enhance the efficiency of electricity generation by ensuring that water droplets flee from condensers – key components in most of the world’s power plants – thus heightening their ability to transfer heat.

Nenad Miljkovic

Nenad Miljkovic

An appropriately charged metal plate positioned close to a condenser can be used to draw leaping droplets away from its surface, preventing the water condensation from returning to the condenser as a result of gravity or the drag created by adjacent vapor flow.

“Now we can use an external electrical field to mitigate [this tendency] and enhance the heat transfer,” said Miljkovic.

Miljkovic says in addition to increasing power plant efficiency via enhanced heat transfer, the jumping droplets could even be used to generate power. Electricity can be generated by placing two parallel metal plates in the open. Droplets would leap from the surface of one of the plates and accumulate on the surface of the other, permitting the generation of power from the simple process condensation formed by the ambient air.

According to Miljkovic, the only prerequisite for this form of power generation is a means of keeping the surface of condensers cool.

“You just need a cold surface in a moist environment,” he said, adding that the team is currently working on the development of the concept.

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