An ‘invisibility cloak’ for tsunamis?

Bryn Nelson Columnist

A tsunami is headed right for a vulnerable shallow-water gas platform. The next minute, the first wave passes by harmlessly as if the structure had completely disappeared. Impossible? Perhaps not, according to a team of French and British physicists that has devised an ‘invisibility cloak’ that could, in theory, hide susceptible platforms or coastlines from ocean waves such as tsunamis.

The solution published last month in the journal Physical Review Letters is earning high marks from some experts for its creativity. However, others are skeptical as to whether the small-scale lab experiment could ever be worked up to ward off a full-scale disaster — such as the Indian Ocean tsunami in that killed more than 200,000 on Dec. 26, 2004.

Sebastian Guenneau, a study co-author, said the experiment’s main focus was to use liquid waves as stand-ins for microwaves to visualize the movement of linear waves that radiate out from a source in a fairly stable pattern — and how they might be disrupted. Initially, Guenneau and physicists from the Centre National de la Recherche Scientifique and Aix-Marseille Universite in France did not consider the potential application for redirecting tsunami waves. But Guenneau said their experimental results suggest an anti-tsunami cloak is well within the realm of possibility.

The cloaking concept is based on positioning multiple rows of pillars at specific intervals within a cylindrical pattern, so that the pillars and intervening spaces resemble a round checkerboard from above. In the lab, a small aluminum cylinder was subdivided into 50 precisely spaced rows of pillars radiating out from a flat center. The columns essentially dissipate oncoming waves so that anything behind the structure is hidden from them.

Maybe even from tsunami waves.

“It’s some kind of crazy idea,” conceded Guenneau, also a lecturer in applied mathematics at the University of Liverpool.

The cloak might work for a smallish island or a structure, such as a coastal nuclear facility, by surrounding it with a semicircular checkerboard pattern of columns, though surrounding much larger land masses would be implausible. More practically, he said, the concept might be used to protect offshore oil and gas platforms.

“You can coat the legs of the platform with this kind of cloak; this will make the legs of the platform, in some sense, invisible to ocean waves,” Guenneau said. “This is an application that is not too crazy.”

Research aimed at bending acoustic, optical and other waves away from specific objects in order to shield them has taken off within the past few years.

Vladimir Shalaev, a professor of electrical and computer engineering at Purdue University, wrote in a review published earlier this month in the journal Science that a new research field known as transformation optics is applying mathematical principles like those in Einstein’s theory of general relativity to produce electromagnetic cloaks that bend light — meaning actual invisibility cloaks may not be far off.

Hiding from waves in plain sight
Whether anti-tsunami cloaks will follow suit remains debatable. Nevertheless, the experiment used several straightforward physics concepts that Guenneau said could be demonstrated even in a high school lab. For their wave generator, the experimenters blew air into a small cube positioned just above the waterline, creating acoustic disturbances that propagated out in concentric rings of small surface waves.

“The acoustic signal is myopic,” Guenneau said. Translation: it doesn’t know that the cloak has multiple rows of pillars and space, like black and white cells in a checkerboard pattern. “It can only see that you have some uniform gray area.”

The concept of adding black and white and ending up with gray, he said, is known in physics as homogenization: “replacing the individual cells with something that behaves the same way, on average.”

To ‘fool’ the acoustic signal into seeing nothing but gray, the researchers cut out their checkerboard-patterned structure from an aluminum cylinder measuring 10 centimeters in diameter (about three inches). Guenneau said any non-porous material, whether rock, wood or cement, could work just as well. The only thing that matters is the periodicity of the pillars, he said.

Water can still flow through the cylinder, unlike the solid wall of a dike designed to stop all water. “You don’t break the wave, you let the wave flow through into your dike,” Guenneau said. “Instead of reflecting the wave, you want to guide the wave to the left and right so the middle zone is protected.”

Incoming waves, Guenneau said, may have many possible points of entry, but the way forward becomes smaller and smaller as they move toward the center of the checkerboard cloak. Consequently, waves lose most of their oomph and much of the water is diverted around the cloak instead, as if it was a solid, imperturbable mass of gray.

In their small-scale laboratory experiment, the physicists found that water was too viscous and unable to flow freely through the cylindrical cloak due to the same phenomenon that sucks up the edges of water in a full drinking glass. As a stand-in, they used a much less viscous liquid known as methoxy-nonafluorobutane, originally formulated as a more environmentally friendly replacement for ozone layer-depleting industrial solvents.

Dan Cox, director of the Hinsdale Wave Research Lab at Oregon State University, expressed doubt over the utility of a scaled-up experiment.

“I don’t see how this work could ever be practically implemented to reduce the threat of tsunamis,” he said in an e-mail. “If you read the article carefully, the authors state ‘we were unable to produce similar results with water.’ ”

But that limitation only exists at the small-scale level, Guenneau argues. For larger-scale applications, he said, the problematic viscosity issue with water is no longer be a constraint.

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