Novel “invisibility cloaks” for water waves leave no telltale wakes or drag

Two separate teams of scientists have devised novel hydrodynamic “invisibility cloaks”—instead of shielding objects from light, the cloacks would shield them from fluid flows. The scientists described their work in two new papers in Physical Review Letters. These kinds of cloaking structures could one day help reduce drag on ships or submarines, or protect ships at a port or wharf from potential damage from strong waves.

Most so-called “invisibility cloaks” created thus far work in the electromagnetic regime and rely on metamaterials. A “metamaterial” is any material whose microscopic structure can bend light in ways light doesn’t normally bend—a property called “the index of refraction.” Natural materials have a positive index of refraction; certain manmade metamaterials—first synthesized in the lab in 2000—have a index of refraction, meaning they interact with light in such a way as to bend light around even very sharp angles.

Metamaterials typically involve a highly conductive metal like gold or copper arranged in carefully layered periodic lattice structures. When light passes through the material, it bends around the cloaked object, rendering it “invisible.” You can see an object directly behind it but can’t see the cloaked object itself. However, the effect is typically limited to specific wavelengths: microwaves, infrared light, or certain frequencies of sound or heat waves.

Earlier this year, Ars reported that a team of French scientists suggested that certain ancient Roman structures, like the famous Roman Colosseum, have very similar structural patterns, which may have protected them from damage from earthquakes over the millennia. So one day metamaterials may be used to lessen the damage to infrastructure from earthquakes by redirecting the more shallow, surface seismic waves that typically inflict the worst structural damage.

There have been prior efforts to construct an “invisibility cloak” for objects in a flowing fluid, which have typically involved micro-pumps or similar active components. What’s unique about a new design from scientists at Seoul National University in South Korea is that it is a passive cloak. A ring of 523 small pillars deflects an incoming fluid flow so that the middle of the ring, where the object or obstacle is—a cylinder, in this case—experiences no drag. The lack of a downstream wake demonstrates that the cylinder is effectively “cloaked.”

A second, independent group of scientists at Beijing University and Xiamen University in China came up with a slightly different passive approach for its hydrodynamic cloak. That’s because the goal of these scientists was not to eliminate drag but reduce the amplitude of water waves within a channel. Inspired by optical waveguide versions of invisibility cloaks, they created their own hydrodynamic version of a gradient index: two thin platforms running along the length to a wave tank’s. This effectively redirects an incoming fluid flow, such that waves in shallower regions propagate more slowly, ultimately leaving the center of the tank free of waves entirely.

DOI: Physical Review Letters, 2019. 10.1103/PhysRevLett.123.074501  (About DOIs).

DOI: Physical Review Letters, 2019. 10.1103/PhysRevLett.123.074502  (About DOIs).

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