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

Enlarge / Hydrodynamic "invisibility cloaks" would hide objects from fluid flows and surface waves so that no wake is visible. (credit: J. Park et al., Phys. Rev. Lett. (2019))

Two separate teams of scientists have devised novel hydrodynamic "invisibility cloaks"-instead of shielding objects from light, the cloaks 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 negative 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.

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