Huang developed a metamaterial that refracts acoustic and elastic waves creating possible applications in super-imaging devices. Credit: Shelby Kardell.
COLUMBIA, Mo. – 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. Now, engineering researchers at the University of Missouri have developed a material that has the ability to control these waves, creating possible medical, military and commercial applications with the potential to greatly benefit society.
“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,” said 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 broadband signals and super-imaging devices also are possibilities.
The material was made in a single steel sheet using lasers to engrave “chiral,” or geometric microstructure patterns, which are asymmetrical to their mirror images (see photo). 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 that will prove its 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 said. “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.”
Going forward, Huang said there are numerous possibilities for the material to control elastic waves including super-resolution sensors, acoustic and medical hearing devices, as well as a “superlens” that could significantly advance super-imaging, all thanks to the ability to more directly focus the elastic waves.
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. The study, “Negative refraction of elastic waves at the deep-subwavelength scale in a single-phase metamaterial,” recently was published in Nature Communications.
Associate professor finds breakthrough in negative elastic wave refraction
Elastic waves are waves that travel on top of or through a material or liquid without causing any permanent changes to the substance’s makeup. Think of sound waves passing through the air, various wave types passing through a body of water, shockwaves from an earthquake, etc. Now, imagine having the ability to exert some control over these waves and the myriad possible applications that could have.
The fabrications were made in a steel sheet with lasers and are chiral microstructures, which means the top and bottom layers are identical in composition but arranged asymmetrically. It’s the first such material to be made out of a single medium. Photo courtesy of Guoliang Huang.
Thanks to Guoliang Huang and his research team, those hypothetical applications are much closer to becoming a reality.
Huang, who joined the MU Mechanical and Aerospace Engineering department as an associate professor in August, recently had a paper published in the journal Nature Communications titled “Negative refraction of elastic waves at the deep-subwavelength scale in a single-phase metamaterial.” The paper details the creation of an elastic metamaterial with engravings that allow for the negative refraction of elastic waves. “This concept, usually you need different materials — metal, rubber and metal again as a combination,” Huang said. “But we designed this material in single-phase material — steel.”
The fabrications were made in a steel sheet with lasers and are chiral microstructures, which means the top and bottom layers are identical in composition but arranged asymmetrically. It’s the first such material to be made out of a single medium.
The artificially engineered material also possesses what Huang called “double-negative properties in elastic media.” This refers to the creation of a negative mass density and bulk modulus in the material, meaning the response of the material to the waves is the opposite of how a material with positive density and bulk modulus would — for example, a material possessing a negative effective bulk modulus supports a volume expansion upon an isotropic compression harmonic loading.
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 with Program Manager Dr. Byung-Lip (Les) Lee. The goal then was to “apply principals of elastic metamaterials for the possibility of controlling and manipulating elastic waves.” Going forward, Huang said there are numerous possibilities with elastic waves and this material, including super-resolution sensors, acoustic devices and a superlens, thanks to the ability to more directly focus the waves.
“Eventually, this can be a tremendous application in structure health monitoring, detecting damage. … This structure now is a passive material,” Huang said. “Now we’re going to make this material active, integrating smart material in these structures. Then we can tune to any frequency. Then we can generate a significant response for any frequency, to make this broadband.”
Huang recently acquired additional funding from the Air Force Office of Scientific Research through 2017, giving him the time and resources necessary to explore the possibilities.