A scaled-up version of this structure could make oil rigs or other ocean structures invisible to tsunami waves (Image: M Farhat/S Enoch/S Guenneau/A B Movchan) North Sea oil rig (Image: Isaac Newton, Wikimedia Commons)

Invisibility cloaks that are able to steer light around two dimensional objects have become reality in the last few years. But the first real-world application of the theories that made them possible could be in hiding vulnerable coastlines and offshore platforms from destructive tsunamis.


The first working invisibility cloak, built in 2006, guided microwaves around a small, flat copper ring as if it wasn’t there. By October 2007, a device repeated the trick for harder-to-handle visible light, and some progress is reported on the yet more complex task of making cloaks to hide 3D objects.

Now Stefan Enoch at the Fresnel Institute in Marseille, France, says that established cloaking principles could be applied to ocean waves, which are essentially two-dimensional.

Such techniques could be used to render vulnerable coastlines or offshore platforms invisible to damaging waves, he says.

Whirlpool effect

To prove it, the researchers have built a prototype 10 centimetres across (see image, right) for testing in a wave tank. Concentric rings of rigid pillars form a labyrinth of radial and concentric corridors.

It may look like waves could pass easily along the radial corridors to the cloak’s centre. But they interact with the pillars, producing forces that pull water along the concentric corridors instead.

“Basically, the cloak behaves like a whirlpool,” says Sebastian Guenneau at the University of Liverpool, UK, and a member of Enoch’s team. “The further you go into the whirlpool, the faster you rotate.”

The spinning rate increases close to the cloak’s centre where the concentric corridors are narrower, making the forces greater, he explains.

As the water whizzes around the cloak, the waves are flung out again along the radial corridors. “If you imagine water entering the cloak from the north, some leaves the cloak to the east, and some leaves to the west, but most is thrown out at the south,” says Guenneau.

The waves exiting the cloak travel as if they have not been disturbed at all, he says.

Huge scale

“I think that this is a great idea with much potential,” says Ulf Leonhardt at the University of St Andrews, UK. “One could really imagine protecting coastlines by arrays of cleverly designed concrete poles.”

Such structures act like metamaterials, materials whose properties result from their structure not composition, and can be used to make invisibility cloaks for light, he says.

But Guenneau cautions that large structures like islands and coastlines are unlikely to become invisible anytime soon, because building the many small islands needed to protect one is such a big job.

“It’s crazy – maybe only people in Dubai could do this,” he adds, referring to the spectacular artificial islands built there.

Smaller structures such as offshore oil platforms would be easier to protect, he says.

Journal reference: Physical Review Letters (DOI: 10.1103/PhysRevLett.101.134501)