New ways to control waves

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.

FLEXIBLE, layered materials textured with nanoscale wrinkles could provide a new way of controlling the wavelengths and distribution of waves, whether of sound or light. The new method was developed by researchers at the Massachusetts Institute of Technology and could eventually find use in applications from non-destructive testing of materials to sound suppression. The findings were described in a paper by Stephan Rudykh and Mary Boyce published recently in the journal Physical Review Letters.

While properties of materials 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 can be as simple as stretching the flexible material. “These effects are highly tunable, reversible, and controllable,” Rudykh said. The materials can be made through a layer-by-layer deposition process 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 sound or light waves. By designing the microstructure of a material 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.”

The research could also provide insights into the properties of natural biological materials, Rudykh said. “Understanding how the waves propagate through biological tissues could be useful for diagnostic techniques,” he said. For example, current diagnostic techniques for certain cancers involve painful and invasive procedures. In principle, ultrasound could provide the same information non-invasively, but today’s ultrasound systems 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, he said.