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chitin Still ... Metamaterials



Riprendiamo il discorso dei metamateriali, cioè di quelle strutture che si comportano in modo non rispondente alla "logica". Per esempio l'opale ha a certe frequenze luminose un indice di rifrazione negativo: in pratica il raggio luminoso esce dal materiale invece di entrarvi ed esservi deviato (come se l'ombra del bastone nell'acqua flottasse nell'aria e vi venisse dispersa). Il risultato finale è un "giocare" con la luce che permette alla superficie di mostrare colori cangianti: tale effetto sfrutta la diffrazione della luce. La diffrazione is the phenomenon that occurs in the vicinity of a crack or a very small barrier (or rather, comparable to the wavelength of light), so that the light creates that particular rainbow effect, such as holograms in credit cards, or watching a CD bias, close to the tracks. We combine
now a second speech, which is that of layering, and self-assembly of materials, as in the shell of crab. Hence the need for protection is combined with the need for visibility, and the surface shows a color which is given purely by constructive interference of light waves with different structure of the material.
Nature has perfect structures, which come to be crystal photonic components, such as frustuli of diatoms, but also layered structures, where instead of the defective (or rather diversity) produces the effect. In the case of elytra of the beetles (which are the "semi-armor" that leave out the wings) of chitin layers with different refractive index (which in fact divert the light in a different way) cause quest'interferenza construction, then the quest'esaltazione color. And the mystery of nature, to think, comes from this wise alternation of order and disorder, or perfection and flaws.

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When using the materials, the nature of the concepts used in that sense "displace" the our basic understanding of their behavior. You can perform tests to characterize the materials and components, but there are some general considerations that are taken for granted before any measurement to be performed. Are common experiences that pulling a rubber band, it is thin, or that a ray of light, penetrating into a material such as glass of a lens, or a liquid such as water, deflected (refracted) into the material itself, or even that a droplet of water on the surface of a material with a more or less spherical, which would allow a degree of cohesion with the surface, so condensarvisi. If you know more about the materials, in particular their electromagnetism, you will also remember other properties: for example in an electrical conductor (eg copper) or a natural magnetic material (ferromagnetic or, for example, iron), the same applied electric or magnetic field, what is transmitted respectively depends on the electric permittivity (ε, epsilon) and magnetic permeability of the material (μ, mu).

However, more recently, it was discovered that there are materials that also have the opposite behavior than the logic: the materials auxetici, we talked about above, and swell when pulled, the film based on the lotus has a negative surface tension and are therefore self-cleaning; opal at certain frequencies has a negative refractive index, then it means that the light beam at certain frequencies out of the material and is rejected (by a circuitous route rather than linear in reality). There are also materials such as in the figure, which have negative electric permittivity and magnetic permeability, so that the electric or magnetic field are reflected in practice, rather than transmitted by the material. This is very different from having the insulation, or dielectric, materials that are only bad conductors, and with high losses (but also with a certain transmission) because it gives you a real total shielding of certain frequencies. These solutions

contrary to the logic design can be generically referred to as metamaterials (beyond the materials), which can also be interpreted as meaning that depend on micro-or nanostructure materials that have, in addition to the material (it is clear that the square spiral material in the figure is the key to its operation as a metamaterial).