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Engineering

Cloaking Ourselves from Disaster

A team at the Institut Fresnel(1) led by Stefan Enoch, in collaboration with researchers in the UK,(2) have developed a novel approach to protect structures from powerful earthquakes and Tsunamis. The secret? Make them invisible.

Listening to Sébastien Guenneau, researcher at Marseille's Institut Fresnel, invisibility is just around the corner. In fact, it already exists, but at wavelengths much longer than our visible spectrum. “Our human eye can only detect electromagnetic waves between 440 nm (purple) and 700 nm (red). But there's a lot more out there, from radio waves (the longest, anywhere between 1 m-1 km) all the way to gamma rays (approximately 0.01 nm). All these electromagnetic waves can be detected because objects either emit them (like stars), or reflect them–the reason you and I see each other. But what if we could make a cloak that would not reflect them, but guide them around an object?” That object would become invisible.
“And this is no longer just theory,” Guenneau explains, referring to the incredible progress made by the physicist John Pendry (Imperial College London) and his colleague David Smith (Duke University), who stipulated in 2004 that for an object to become invisible to a specific electromagnetic wave, it would have to be surrounded by a material whose structures were ten times smaller than the wave itself. Though it is still impossible to tackle visible light (the structures would have to be 10 nm), specially-built metameterials3–as they are known–could be fine-tuned to tackle microwaves (with structures in the millimeters). Two years later, Pendry and Smith were able to shield an object from microwaves of a certain frequency by placing it at the center of a metamaterial (12 cm in diameter) made up of a system of concentric rings: the first successful invisibility cloak.
“We realized that the principle would hold no matter the type of wave: electromagnetic, acoustic, or mechanical, like waves moving on the surface of a liquid,” explains Guenneau. In 2008, the team grabbed headlines with the promise of an anti-tsunami shield4 which would manipulate ocean waves to hide a central object. In the lab, they created a metamaterial that consisted of an aluminum disc 20 centimeters in diameter covered by 7 concentric rows of pillars surrounding a flat center, and submerged under the surface of a liquid. Waves that entered the maze would bounce off the pillars, cancel each other out, and end up traveling around the “corridors” to exit on the opposite end of the disc, leaving waters in the “eye” of the disc relatively calm. The scientists believe it would be possible to replicate this type of structure to shield oil platforms, and possibly entire coastlines from tsunamis and severe storms, though at this scale, cost could become prohibitive.
cloaking ourselves

© S. Enoch

Large-scale models of this small structure (10 cm in diameter) could make entire coastlines or offshore platforms “invisible” to tsunamis.



But Guenneau and his colleagues didn't stop there. Their current research5 tackles seismic waves. In the lab, they devised an elastic “thin plate” on which sits a cylinder 120 cm in diameter and 1 cm in height representing the object to shield. The metamaterial is in this case made up of six different types of plastic polymers arranged concentrically around the cylinder, each tuned to a specific wavelength. Vibrations hitting the plate are deflected by this metamaterial, leaving the central cylinder untouched. And if the model currently only works with surface waves–the most destructive–theoretical work is in progress to bend coupled pressure and shear body waves as well,”6 he explains, stating that they are in the process of filing a patent with CNRS for this elastic cloak, which would have potential applications ranging from the car industry to aeronautics and anti-earthquake systems. From cloaking stadiums from seismic waves to protecting oil platforms from tumultuous seas, invisibility has never looked so promising.

Saman Musacchio

Notes :

1. Laboratoire CNRS / Universités Aix-Marseille-I and -III / École Centrale Marseille.
2. Alexander Movchan, Division of Applied Mathematics (University of Liverpool, UK).
3. These are materials not found in nature, but built from one or more materials, or by modifying existing materials, to answer a specific function. They can be engineered using chemistry, physics, or mechanics.
4. M. Farhat et al., “Broadband Cylindrical Acoustic Cloak for Linear Surface Waves in a Fluid,” Phys. Rev. Lett., 2008. 101: 134501.
5. M. Farhat et al., “Ultrabroadband Elastic Cloaking in Thin Plates,” Phys. Rev. Lett., 2009. 103: 024301.
6. M. Brun et al., “Achieving control of in-plane elastic waves,” App. Phys. Lett., 2009. 94: 061903.

Contacts :

Institut Fresnel, Marseille.
Sébastien Guenneau,
sebastien.guenneau@fresnel.fr
Stefan Enoch,
stefan.enoch@fresnel.fr


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