During the past few years, the transformation-optics approach has been also successfully applied to the manipulation of a variety of other different physical quantities like acoustic and elastic waves, static magnetic and electric fields, and heat flux, in view of the mounting interest towards metamaterials beyond electromagnetics.
Although it seems suggestive to coherently exploit these various approaches to design multifunctional devices capable of performing multiple functions in different physical domains, most studies available in the literature deal with the design of a single functionality in a given physical domain.
In collaboration with Yuki Sato’s Group (Harvard University), we have recently introduced an approach to the design of metamaterial shells capable of governing different physicals phenomena in an independent fashion [1].
From the mathematical viewpoint, our relies on the well-known coordinate-transformation framework. Viewing the rerouting of energy flow as a distortion of space from a coordinate transformation, this technique provides a powerful and systematic recipe for designing and fabricating artificial structures that can mold the flow of a given physical quantity in a desired fashion.
Conventionally, this theory is applied to a single physical domain. In our approach, we instead exploit separate but intertwined coordinate transformations to simultaneously manipulate multiple physical phenomena in independent fashions. Thus, for instance, a material may be designed to exhibit a particular thermal functionality while its electrical functionality is made drastically different.
As a proof-of-concept example, we have synthesized and designed a metamaterial shell that behaves simultaneously as a thermal concentrator and an electrical invisibility cloak. In such a shell, the thermal and electrical currents follow markedly different paths, with the former concentrating in the inner region and the latter circumventing that region, as schematized in the figure top panel. The bottom panel shows instead the thermal and electrical responses, which exhibit the expected behaviors.
Although the results of this study are based on theory and numerical simulations, they do indicate that practical realization of these metamaterials should be within reach of current fabrication technologies. Our numerical simulations indeed show that they can be fabricated by means of small sub-blocks made of realistic materials, such as aluminum nitride and silver conductive epoxies.
Such design may be considered as a first step towards a more general framework that could be defined as “transformation-multiphysics”, which may be in principle extended and applied to a variety of electrical, magnetic, acoustic, and thermal systems in various combinations, and may be integrated in advanced materials, spanning multiple orders of magnitude in material scales.
Besides serving as a proof-of-principle, the independent manipulation of electrical and thermal currents is also very important from the application viewpoint. One particularly intriguing application is the engineering of thermoelectric materials, which hold great promises for energy recycling, by converting waste heat (e.g., generated by the CPU of a computer) into additional electrical power. For these materials, the independent control of electrical and thermal conductivities is of paramount importance, and it may affect the figure of merit in ways unexplored in the past.
Spatial tailoring of the material constitutive properties is a well-known strategy to mold the local flow of given observables in different physical domains. Coordinate-transformation-based methods (e.g., transformation optics) offer a powerful and systematic approach to design anisotropic, spatially inhomogeneous artificial materials (metamaterials) capable of precisely manipulating wave-based (electromagnetic, acoustic, elastic) as well as diffusion-based (heat) phenomena in a desired fashion. However, as versatile as these approaches have been, most designs have thus far been limited to serving single-target functionalities in a given physical domain. Here, we present a step towards a “transformation multiphysics” framework that allows independent and simultaneous manipulation of multiple physical phenomena. As a proof of principle of this new scheme, we design and synthesize (in terms of realistic material constituents) a metamaterial shell that simultaneously behaves as a thermal concentrator and an electrical “invisibility cloak.” Our numerical results open up intriguing possibilities in the largely unexplored phase space of multifunctional metadevices, with a wide variety of potential applications to electrical, magnetic, acoustic, and thermal scenarios.
@article{IJ112_PRX_4_021025_2014, title = {Independent manipulation of heat and electrical current via bifunctional metamaterials}, author = {Moccia, Massimo and Castaldi, Giuseppe and Savo, Salvatore and Sato, Yuki and Galdi, Vincenzo}, journal = {Physical Review X}, volume = {4}, issue = {2}, pages = {021025}, numpages = {14}, year = {2014}, month = may, publisher = {American Physical Society}, doi = {10.1103/PhysRevX.4.021025}, url = {http://link.aps.org/doi/10.1103/PhysRevX.4.021025} }