Elastic Cloaking Material Makes Objects “Unfeelable”

Move over, invisibility cloaks. There's a structure that can keep objects from being felt or jostled
unfeelable cloak

With the finger or a force measurement instrument, no information is obtained about the bottom side of the material.
Credit: T Bückmann / KIT

Invisibility cloaks, once thought of as the province only of Harry Potter tales and Star Trek, have become reality in the past decade or so. Now scientists have developed an "unfeelability cloak," a material that hides objects within it from being felt or touched. The researchers suggest that in the future such cloaks might find help protect objects from bumps and pokes that might otherwise harm them.

Invisibility cloaks work by smoothly guiding light waves around objects so the waves ripple along their original trajectories as if nothing were there to block them. Scientists have designed cloaking materials that work against other kinds of waves as well—for example, inaudibility cloaks hide objects from the acoustic waves used in sonar.

The unfeelability cloak is a so-called pentamode metamaterial, an artificial structure that, despite being a solid, can behave like a fluid; although difficult to compress, its shape is otherwise easy to shift. The specific material the researchers devised is a three-dimensional hexagonal lattice reminiscent of a honeycomb, with the rods making up this lattice wider at their middles than at their ends. "This is the first experimental example of an elastomechanical unfeelability cloak, and one of the first applications demonstrated for pentamode materials," says mathematician Graeme Milton at the University of Utah, who did not take part in this research.

The scientists built the cloak around the object they intended to hide, a rigid hollow cylinder that itself could hide anything big enough to fit within it. To simultaneously create both the cloak and the cylinder, the researchers shone an infrared laser beam into a vat of light-sensitive fluid, with plastic shapes hardening where the laser beam had focused. The resulting cloak was made of rods only about 40 microns long and 10 microns thick at their widest points, built around a cylinder 750 microns in diameter with walls 125 microns wide. (In comparison, the average human hair width is roughly 100 microns.) "Creating the cloak was physically challenging—the components can sometimes be only 3.3 to 6.6 microns thick, and their dimensions have to be very precise for the cloak to work, so developing a technique to fabricate the cloak was difficult," says lead study author Tiemo Bückmann, a physicist at the Karlsruhe Institute of Technology in Germany. The scientists detailed their findings online June 19 in the Nature Communications. (Scientific American is part of Nature Publishing Group.)

The widths and lengths of the components making up the cloak are calculated so that they will surround and hide a cloaked object of particular dimensions from scientific instruments or even fingers. The cloak essentially deforms around the object it hides, making it feel as if the object were not there; at the same time, the object is shielded from anything touching it. "I found it very astonishing how good the cloak works," Bückmann says.

When regular carpets or mattresses are placed on top of bumpy surfaces, the hidden bumps can sometimes still be felt, "like in Hans Christian Andersen's fairy tale, The Princess and the Pea," Bückmann says. In contrast, with this new unfeelability cloak, "it's very interesting to me how little material one has to put above the obstacle in order to cloak the object," he says.

The unfeelability cloak does have limits. For instance, the dimensions of the cloak's components have to be designed to specifically match whatever the cloak is hiding. Furthermore, although unfeelability cloaks can hide objects from some pokes and prods, it cannot protect against all of them—one can push on a cloak strongly enough to break it and feel the object it hides. Still, cloaking an object from touch could have many uses, Bückmann says. "One could use the technology whenever one needs to prevent a material from being harmed, such as preventing a parcel from being crushed," Bückmann says. Other potential applications might include camping mattresses or carpets that are very thin, light and comfortable but can still hide rocks or cables underneath them.

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