Bending acoustic, elastic waves with metamaterials

Sound waves passing through the air, objects that break a body of water and cause ripples, or shockwaves from earthquakes – all are considered elastic waves. These waves travel at the surface or through a material without causing any permanent changes to the substance’s makeup. A new material developed by engineering researchers at the University of Missouri (MU) can control these waves, creating possible medical, military, and commercial applications.

“Methods of controlling and manipulating subwavelength acoustic and elastic waves have proven elusive and difficult; however, the potential applications – once the methods are refined – are tremendous,” says Guoliang Huang, associate professor of mechanical and aerospace engineering in the College of Engineering at MU. “Our team has developed a material that, if used in the manufacture of new devices, could have the ability to sense sound and elastic waves. By manipulating these waves to our advantage, we would have the ability to create materials that could greatly benefit society – from imaging to military enhancements such as elastic cloaking. The possibilities truly are endless.”

In the past, scientists have used a combination of materials such as metal and rubber to effectively bend and control waves. Huang and his team designed a material using a single component: steel. The engineered structural material possesses the ability to control the increase of acoustical or elastic waves. Improvements to super-imaging devices and broadband signals also are possible.

The material was made in a single steel sheet using lasers to engrave chiral patterns – geometric microstructures – that are asymmetrical to their mirror images. It’s the first such material to be made out of a single medium. Huang and his team intend to introduce elements they can control to prove the material’s usefulness in many fields and applications.

“In its current state, the metal is a passive material, meaning we need to introduce other elements that will help us control the elastic waves we send to it,” Huang says. “We’re going to make this material much more active by integrating smart materials like microchips that are controllable. This will give us the ability to effectively ‘tune in’ to any elastic sound or elastic wave frequency and generate the responses we’d like. This manipulation gives us the means to control how it reacts to what’s surrounding it.”

The research began five years ago during Huang’s tenure at the University of Arkansas-Little Rock and was funded by a grant from the U.S. Air Force Office of Scientific Research. Byung-Lip (Les) Lee served as program manager.

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