'Mechanical' Invisibility Cloak Inspired by the Honeycomb

Honeycombs are usually stable structures, but what happens if there's a hole?. Sean.hoyland [public domain]/Wikimedia

The mechanical structure of a honeycomb is among the most stable found in nature. The hexagonal design allows for an efficient, secure lattice. But what happens when there are imperfections in that lattice, such as when a hole forms? The honeycomb structure can be extremely weakened.

With the ultimate aim of designing new construction materials that can remain relatively stable in spite of such a hole, researchers at the Karlsruhe Institute of Technology (KIT) have developed a "mechanical" invisibility cloak of sorts, which is capable of masking any imperfections found in the classic honeycomb, according to a KIT press release. This will eventually allow the researchers to develop strong materials in spite of the recesses.

The method makes use of "coordinate transformation," which is essentially a distortion made to a lattice by bending or stretching it. For light, such transformations are based on the mathematics of transformation optics, which is also the rhyme behind the reason of how invisibility cloaks work. So far, however, it has been impossible to transfer this principle to real materials and components in mechanics because the mathematics simply don't apply to the mechanics of actual materials.

But the new method developed by KIT researchers is capable of overcoming these difficulties.

"We imagined a network of electric resistors," explained Tiemo Bückmann, lead author of the study. "The wire connections between the resistors may be chosen to be of variable length, but their value does not change. Electric conductivity of the network even remains unchanged, when it is deformed."

"In mechanics, this principle is found again when imagining small springs instead of resistors. We can make single springs longer or shorter when adapting their shapes, such that the forces between them remain the same. This simple principle saves computation expenditure and allows for the direct transformation of real materials."

Basically, by applying this method to a honeycomb structure with a hole, researchers were able to reduce the error or 'weakness' of the structure down from 700 percent to just 26 percent. It's a remarkable transformation, one that could lead to materials that appear deformed, but which are nevertheless capable of reacting stably against external forces-- as if the structure was not deformed. It's in this way that the deformity is merely made into a mechanical illusion. Imagine the fun architects could have with this!

The results have just been published in the Proceedings of the National Academy of Sciences (PNAS).