JK Rowling might have contemplated optical invisibility, but scientists at the Karlsruhe Institute of Technology (KIT), Germany, claim to have mimicked magic a notch ahead. While Rowling imagined an invisibility cloak that prevented the subject from being seen, the KIT team claims they can prevent the subject from being felt, or even sensed by a force measuring device. This can be considered as a new realm – Unfeelability or Mechanical Invisibility.
To most people, invisibility cloaks often closely relate to hiding objects from the sense of vision. However, invisibility cloaks might also refer to technologies that hide objects from one of our other senses. Inventions in the past could hide objects from light, heat or sound. Till date, cloaking has been experimentally demonstrated in many fields, including electrodynamics at microwave frequencies, optics, static electric conduction, acoustics, fluid dynamics, thermodynamics and quasi two-dimensional solid mechanics. However, concealing an object from being touched still remained to be unaccomplished.
Harry Potter’s invisibility cloak, as interesting as it might seem, did not interfere with our sense of touch, i.e someone could still feel the subject even though he/she was concealed from vision. However, the KIT team has produced a polymer pentamode metamaterial with sub-micrometer accuracy that is capable of shielding objects placed inside it such that they appear not to be there at all. U.S Patent application US20140126322 discloses a similar method where pentamode metamaterial structures propagate waves according to pentamode elastic theory.
Scientist Tiemo Bückmann and his colleagues at KIT have been successful in creating an invisibility cloak based on crystalline material of long, thin cones, whose tips meet. The size of the contact points of the cones, when adjusted appropriately, can result in a structure that can fool the senses to an extent that a finger or a measurement instrument cannot feel its way through. In the invisibility cloak produced, a hard cylinder is inserted into the bottom layer. Objects that one intends to hide from touch can be put into its cavity. The scientists have built a structure that can surround the object to be hidden. In this structure, strength depends on the location in a defined way. (U.S Patent US8833510 published on Sep 16, 2014 describes designing structured metamaterials for isolating an entity from external mechanical vibrations. Further, U.S Patent Application US20130241123 filed on Sep 13, 2011 discusses metamaterials for bending elastic waves around a cylindrical zone.)
The metamaterial structure directs the forces of the touching finger in such a way that the cylinder is completely ‘invisible’ to touch. 3D dip-in direct-laser-writing optical lithography has allowed for the fabrication of such mechanical metamaterial architectures with submicron features yet cubic millimeter overall volumes at the same time.
The aim is to statically cloak an arbitrary object located inside a rigid (ideally incompressible and not deformable) hollow cylinder with inner radius Ri, outer radius R1, bulk modulus B1 –> ∞ and shear modulus G1 –> ∞. The rigid wall already isolates an object inside with respect to the outside. By additionally wrapping around the cylinder, a homogeneous isotropic shell (with outer radius R2, bulk modulus B2 and shear modulus G2), a core-shell geometry can be made to appear elastically as its isotropic compliant homogeneous surrounding (with bulk modulus B0 and shear modulus G0). The core shell should appear same as the surrounding with respect to both – compression and shear. This requires G1= G0, from which the condition G2=G1=G0 follows. A rigid isolating wall (with B1 –> ∞ and G1 –> ∞) requires a solid. This leads to a surrounding with G0 –> ∞, which is not deformable as well. This poses a limitation to represent cloaking. The mechanical core-shell approach does not allow for perfect elasto-mechanical cloaking using a rigid wall for isolation and a compliant solid as surrounding. The scientists at KIT tried an approximate approach based on a rigid wall, where the surrounding as well as the shell exhibit relatively small shear moduli. The conditions for bulk and shear modulus of the shell can be mapped onto the properties of a pentamode microstructure.
Though the KIT team has created the cloak as part of fundamental physics research without any specific applications in mind, they suggest it might open doors to interesting applications in coming years, as it allows for producing materials with freely selectable mechanical properties. For instance, it would be now possible to make carpets that hide cables running beneath them, or light and thin camping mattresses that can protect against rocky ground or stiff tree roots.
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