It is possible to engineer a material so that it gives a desired response to an incoming wave — such as the metamaterials that can manipulate electromagnetic waves to create 'invisibility cloaks'. The same idea is equally applicable to mechanical waves or sound. Muamer Kadic and co-workers have now devised a metamaterial that could be the basis of an acoustic cloak.
In 1995, scientists showed theoretically that an array of structures known as pentamodes could create a metamaterial with any conceivable mechanical property, with each pentamode comprising eight cones placed end to end. But a structure that balances cones point-to-point is obviously unstable, and so such a metamaterial has proven impossible to realize in practice.
Kadic et al. show that modifying this idea so that the cones are slightly truncated gives a structure that is both producible and has the required mechanical properties. They created the material from a block of polymer using a process known as direct-laser-writing optical lithography. The technique is sensitive enough to produce pentamodes with a truncated-end diameter as small as 0.55 micrometres.
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Gevaux, D. Structurally sound. Nature Phys 8, 442 (2012). https://doi.org/10.1038/nphys2345
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DOI: https://doi.org/10.1038/nphys2345