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Cold water is in the hot seat. Usually, cool water moves away from hot areas – but make the droplets small enough and surprisingly, the opposite happens. The effect may help us cool electronic devices, with tiny droplets of water propelling themselves to the hotter areas before evaporating and cooling the surface.

Fluids tend to move when there is an imbalance of surface tension – the force that helps you blow soap bubbles. Temperature can cause surface tension to change, with fluids generally flowing away from warmth.

But get down to the nanoscale and the effect is reversed. Using computer simulations, Suman Chakraborty at the Indian Institute of Technology in Kharagpur and his colleagues have found that nanometre-sized water droplets on water-repellant surfaces move towards the heat.

“This traditional manifestation of droplet motion from high temperature to low temperature can be reversed,” says Chakraborty.

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