Metamaterials, which can morph according to their environment, make up a new class of 3D printable, engineered surfaces which can perform nature-defying tasks, like making holograms and shaping sound. Recently, a collaborative team led by researchers from the USC Viterbi School of Engineering created new 3D printed acoustic metamaterials that are able to be remotely switched on and off, using a magnetic field, between active control and passive states.
This makes it possible to control vibration and sound, which other researchers have been trying unsuccessfully to do with abnormal property-exhibiting structures. The difference is that those metamaterials are built in fixed geometries, so their abilities will also remain fixed.
Wang, together with USC Viterbi PhD student Kun-Hao Yu, University of Missouri Professor Guoliang Huang, and MIT Professor Nicholas X. Fang, whose work with 3D metamaterials we’re familiar with, have developed 3D printed metamaterials that can block both mechanical vibrations and sound waves. This opens up applications in vibration control, noise cancellation, and sonic cloaking (used to hide objects from acoustic waves), because, unlike current metamaterials, these can be controlled remotely with a magnetic field.
“When you fabricate a structure, the geometry cannot be changed, which means the property is fixed,” explained Qiming Wang, USC Viterbi Assistant Professor of Civil and Environmental Engineering. “The idea here is, we can design something very flexible so that you can change it using external controls.”
Yu said, “Traditional engineering materials may only shield from acoustics and vibrations, but few of them can switch between on and off.”
Yu, Fang, Huang, and Wang, whose research was funded by the National Science Foundation and the Air Force Office of Scientific Research Young Investigator Program, recently published a paper, titled “Magnetoactive Acoustic Metamaterials,” in the Advanced Materials journal.
The abstract reads, “In conventional acoustic metamaterials, the negative constitutive parameters are engineered via tailored structures with fixed geometries; therefore, the relationships between constitutive parameters and acoustic frequencies are typically fixed to form a 2D phase space once the structures are fabricated. Here, by means of a model system of magnetoactive lattice structures, stimuli‐responsive acoustic metamaterials are demonstrated to be able to extend the 2D phase space to 3D through rapidly and repeatedly switching signs of constitutive parameters with remote magnetic fields. It is shown for the first time that effective modulus can be reversibly switched between positive and negative within controlled frequency regimes through lattice buckling modulated by theoretically predicted magnetic fields.”
Metamaterials can manipulate wave phenomena, like light, radar, and sound, which helps create technology like cloaking devices. Environmental sounds and structural vibrations, which have similar waveforms, can now be controlled by the team’s unique metamaterials. These can be compressed, but not constrained, with a magnetic field by 3D printing a deformable material, which contains iron particles, in a lattice structure. So, when a mechanical or acoustic wave makes contact with the 3D printed metamaterial, it disturbs it, which then produces the properties that can block certain frequencies of mechanical vibrations and sound waves.
“You can apply an external magnetic force to deform the structure and change the architecture and the geometry inside it,” said Wang. “Once you change the architecture, you change the property. We wanted to achieve this kind of freedom to switch between states. Using magnetic fields, the switch is reversible and very rapid.”
In order to work, the mechanism needs the negative modulus and density of the metamaterials; these are both positive in regular materials. An object will typically push back against you if you push it, but objects with a negative modulus pull you forward as you push; objects with negative density move toward you when you push them.
Yu explained, “Material with a negative modulus or negative density can trap sounds or vibrations within the structure through local resonances so that they cannot transfer through it.”
Just one negative property, be it density or modulus, is able to independently block vibrations and noise within certain frequencies, but these can pass through if the two negative properties work together. By switching the magnetic field, the researchers have versatile control and can switch the metamaterial between double-positive (sound passing), single-negative (sound blocking), and double-negative (sound passing again).
Wang said, “This is the first time researchers have demonstrated reversible switching among these three phases using remote stimuli.”
The team’s current system can only 3D print metamaterials with beam diameters between one micron and one millimeter, so it either needs to grow or shrink. Larger beams would affect lower frequency waves, while smaller ones would control waves of higher frequencies.
“There are indeed a number of possible applications for smartly controlling acoustics and vibrations. Traditional engineering materials may only shield from acoustics and vibrations, but few of them can switch between on and off,” Yu said.
Now, Wang thinks the team could get their metamaterial to demonstrate another unique property – negative refraction, or “anti-physics,” where a wave goes through the material and comes back in at an unnatural angle. Once the researchers manage to 3D print larger structures, they’ll focus more on studying this phenomenon.
“We want to scale down or scale up our fabrication system. This would give us more opportunity to work on a larger range of wavelengths,” Wang said.
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