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Hot bodies get super-slippery when wet

Steamy balls could lead to sizzling speedy torpedoes

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An Australian boffin says he has come up with a novel method for making things such as ship's hulls or torpedoes slip through water more easily.

Professor Derek Chan of Melbourne uni suggests that it would be practical for ships to exploit the "Leidenfrost effect", named after its discoverer in 1756. This refers to the behaviour of liquids when they encounter a surface significantly hotter than their boiling point: the portion of the liquid in contact turns into a cushion of vapour, keeping the remainder of the liquid away from the hot surface.

An everyday example of the Leidenfrost effect in play is that of a drop of water hitting a hot skillet and skittering about. As the vapour cushion is poorer at transmitting heat than direct contact, the droplet will then actually take longer to boil away than it would have if the pan were cooler. The same effect can also allow a person to put their hand into a bucket of liquid nitrogen without harm.

The Leidenfrost effect is poorly understood, however: it's quite hard to predict at just what temperature it will set in. But Professor Chan has carried out detailed experiments which involved dropping hot, polished balls into various liquids and viewing their interactions on high-speed video. The prof believes he's gained enough of a handle on the effect that it could be used in practical applications.

"An obvious area of application is shipping," he says. "Australia transports a large amount of products such as iron ore and grain around the world. The ship's hot body could substantially minimise the amount of drag as it passes through water, therefore potentially reducing transportation costs and greenhouse gas emissions."

In effect, the situation would be the same as that of the water droplet on the hot skillet but reversed: the hot ship would skitter frictionlessly on the sea's surface just as the drop does on the pan. However the prospect of massive bulk carriers with their hulls heated up well past boiling point seems a trifle unrealistic.

Where the idea might find an application is in torpedos, whose speed is seriously limited by water drag. There is already an advanced Russian rocket torpedo – the famous "Shkval" – which operates on similar lines, though the Shkval generates its slippery surrounding gas layer internally rather than by evaporating water. Leidenfrost-effect hot projectiles might offer similar high speeds.

That said, "there are still a number of issues that need to be addressed before this drag reduction method can be applied commercially," warns Chan, "such as the effect of increased heat on corrosion.”

The research is laid out in a paper by Chan and his colleagues, published in the journal Physical Review Letters. ®

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