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Nanoscale wrinkles help improve sound wave control

29 Jan 2014  | David L. Chandler

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Massachusetts Institute of Technology researchers have designed a technique to modify the properties of layered biological materials so that sound or light waves can be controlled. A microstructure created through this process could be useful in various fields, such as medical diagnostics and consumer electronics. Published in the journal Physical Review Letters, the study received support from the U.S. Army Research Office through the MIT Institute for Soldier Nanotechnologies.

While materials' properties are known to affect the propagation of light and sound, in most cases these properties are fixed when the material is made or grown, and are difficult to alter later. But in these layered materials, changing the properties—for example, to "tune" a material to filter out specific colours of light—can be as simple as stretching the flexible material.

"These effects are highly tunable, reversible, and controllable," according to study co-author Stephan Rudykh. "For example, we could change the colour of the material, or potentially make it optically or acoustically invisible."

Nanoscale wrinkles help improve sound wave control

Figure 1: In the top pair of images, sound waves (blue and yellow bands) passing through a flat layered material are only minimally affected. In the lower images, when sound goes through a wrinkled layered material, certain frequencies of sound are blocked and filtered out by the material. Image credit: Felice Frankel

The materials can be made through a layer-by-layer deposition process, refined by researchers at MIT and elsewhere, that can be controlled with high precision. The process allows the thickness of each layer to be determined to within a fraction of a wavelength of light. The material is then compressed, creating within it a series of precise wrinkles whose spacing can cause scattering of selected frequencies of waves (of either sound or light). The research team found that these effects work even in materials where the alternating layers have almost identical densities. "We can use polymers with very similar densities and still get the effect," Rudykh added. "How waves propagate through a material, or not, depends on the microstructure, and we can control it."

By designing that microstructure to produce a desired set of effects, then altering those properties by deforming the material, "we can actually control these effects through external stimuli," Rudykh said. "You can design a material that will wrinkle to a different wavelength and amplitude. If you know you want to control a particular range of frequencies, you can design it that way."

The research, which is based on computer modelling, could also provide insights into the properties of natural biological materials. It could improve ultrasound systems in clinical diagnostics, where noninvasive techniques for certain cancers currently lack sufficient resolution. The new work with wrinkled materials could lead to more precise control of these ultrasound waves, and thus to systems with better resolution.

The system could also be used for sound cloaking—an advanced form of noise cancellation in which outside sounds could be completely blocked from a certain volume of space rather than just a single spot, as in current noise-cancelling headphones.

"The microstructure we start with is very simple," Rudykh says, and is based on well-established, layer-by-layer manufacturing. "From this layered material, we can extend to more complicated microstructures, and get effects you could never get" from conventional materials. Ultimately, such systems could be used to control a variety of effects in the propagation of light, sound, and even heat.

The MIT team has applied for intellectual property rights to the technology and is in discussions with companies to commercialise the discovery.




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