Natural granular pile as electromagnetic ground cloak

Rendering an object invisible through a cloaking device is an ongoing dream of human beings and material scientists. Recently, intensive theories and experiments have predicted and demonstratedthat such a cloaking device can be realized based on transformation optics and metamaterials, in fields of electromagnetics, optics acoustics, or even heat transfer. Metamaterials enable precise control over the propagation of waves due to their delicate micro-structures and spatially tailorable properties. However, a simple and natural way to achieve cloaking without a delicate micro-structure remains unattainable. Here, we report that an electromagnetic quasi-cloaking device can be readily achieved by a granular pile, formed by the falling-off and cumulation of particles on the object to be cloaked. We show that natural particle size segregation during the formation of the granular pile imparts the pile with a spatially variable filling ratio and resultant gradient-distributed permittivity, which enables the realization of the electromagnetic cloaking device. This work may open the possibility that a complicated electromagnetic device may be alternately achieved from manipulation of particle movement or arrangement.