Sound waves could be used to remotely reprogram material stiffness, study shows
A team of researchers co-led by the University of California San Diego, University of Michigan, and the French National Center for Scientific Research (CNRS) at Laboratory of Acoustics of Le Mans University has demonstrated a new way to remotely control how a material behaves — using sound. The findings could lead to the development of protective gear, robotic muscles or medical implants that adjust their stiffness on demand.
In a study published in Nature Communications, the team showed for the first time that specific frequencies of acoustic waves can be applied to a material to reliably move localized features known as mechanical kinks, which determine whether different regions of the material are soft or stiff.
Kinks act as boundaries between two distinct internal states of a material. On either side of a kink, the material may be made of the same atoms or building blocks, but those blocks are oriented differently in three dimensions. This subtle change can lead to very different mechanical properties. Mechanical kinks, in particular, mark where a material deforms. They appear, for example, where metals permanently bend or where DNA strands separate.
Materials scientists have long been interested in controlling kinks because moving one can reshape how a material behaves. For instance, it can alter which parts of a material feel soft versus stiff. But doing so has proven difficult. In most materials, kinks encounter energy barriers that pin them in place. And while previous studies have shown it is possible to move kinks using sound waves, the resulting motion was typically chaotic and difficult to predict, explained study co-corresponding author Nicholas Boechler, a professor in the Department of Mechanical and Aerospace Engineering at the UC San Diego Jacobs School of Engineering.
In collaboration with study co-corresponding authors Xiaoming Mao at University of Michigan and Georgios Theocharis at CNRS at Laboratory of Acoustics of Le Mans University, Boechler and colleagues have shown a way to move a kink in a controlled manner using sound waves. They modeled a material in which moving the kink costs no energy, which is an unusual and rare property. This was achieved by designing a material whose behavior is dictated by its structure rather than its composition.
In the model material, wherever the kink is located, that region is soft, while the rest of the material grows progressively stiffer. If the kink is moved to one end, that end becomes soft while stiffness increases exponentially toward the opposite end. Move the kink to the other side, and the stiffness profile flips. Move the kink to the middle, and the material becomes soft in the center and stiff toward both ends.
“The idea here is that we’ve essentially made an acoustic tractor beam that moves a kink and changes the way a material feels — while creating gradients of stiffness — on demand,” Boechler said.
Because the model material has no energy barriers, the researchers were able to use sound waves not just to move the kink, but do so predictably and step by step.
“We showed that if you send acoustic waves in from one side, they actually pull the kink toward where the sound came from,” Boechler said. “You can send a small pulse, and the kink moves a little. Send another pulse, and it moves a little more. It’s basically remote control for the material’s internal state.”
To demonstrate, the team built a life-sized experimental model consisting of a chain of stacked, rotating disks connected by springs. Each disk represents an individual atom, while the springs mimic atomic bonds. One disk — arranged differently from the rest — represents the kink. When short pulses of acoustic waves were sent into the structure, the kink was pulled toward the sound source, moving a few disks at a time. Each additional short burst of vibration nudged the kink a little farther. When longer vibrations were applied, the kink was continuously pulled across the entire length of the chain, effectively flipping which side of the chain was soft and which was stiff.
While the researchers can currently pull the kink rather than push it, they note that the level of control already surpasses anything achieved previously. The team also demonstrated that only certain sound frequencies cause the kink to move, while others have no effect.
Computer simulations also revealed that when a sound wave reaches the kink, part of the wave reflects and part passes through. Even so, the interaction transfers momentum to the kink and allows it to continue moving.
The system modeled in this study points to potential future applications such as materials with tunable stiffness, shape-changing structures and robust signal transmission.
“Right now, this is a toy model,” Boechler noted. “If something like this could be made into a real material, you could imagine structures that adapt on the fly — materials you can reprogram using sound.”
Next steps include exploring three-dimensional versions of the model system and investigating whether similar effects could exist at much smaller, even atomic, scales.
“This is fundamental research,” Boechler said. “But fundamental discoveries are often what end up advancing technology in the long run. Our work shows what becomes possible when you design materials with genuinely new properties.”
Full study: “Observation of mechanical kink control and generation via acoustic waves.” Additional study co-authors include Kai Qian (first author) and Nicolas Herard, UC San Diego; and Nan Cheng, Francesco Serafin and Kai Sun, University of Michigan.
This work was supported by the U.S. Army Research Office (grant W911NF-20-2-0182) and the U.S. Office of Naval Research (MURI N00014-20-1-2479).
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In a study published in Nature Communications, the team showed for the first time that specific frequencies of acoustic waves can be applied to a material to reliably move localized features known as mechanical kinks, which determine whether different regions of the material are soft or stiff.
Kinks act as boundaries between two distinct internal states of a material. On either side of a kink, the material may be made of the same atoms or building blocks, but those blocks are oriented differently in three dimensions. This subtle change can lead to very different mechanical properties. Mechanical kinks, in particular, mark where a material deforms. They appear, for example, where metals permanently bend or where DNA strands separate.
Materials scientists have long been interested in controlling kinks because moving one can reshape how a material behaves. For instance, it can alter which parts of a material feel soft versus stiff. But doing so has proven difficult. In most materials, kinks encounter energy barriers that pin them in place. And while previous studies have shown it is possible to move kinks using sound waves, the resulting motion was typically chaotic and difficult to predict, explained study co-corresponding author Nicholas Boechler, a professor in the Department of Mechanical and Aerospace Engineering at the UC San Diego Jacobs School of Engineering.
In collaboration with study co-corresponding authors Xiaoming Mao at University of Michigan and Georgios Theocharis at CNRS at Laboratory of Acoustics of Le Mans University, Boechler and colleagues have shown a way to move a kink in a controlled manner using sound waves. They modeled a material in which moving the kink costs no energy, which is an unusual and rare property. This was achieved by designing a material whose behavior is dictated by its structure rather than its composition.
In the model material, wherever the kink is located, that region is soft, while the rest of the material grows progressively stiffer. If the kink is moved to one end, that end becomes soft while stiffness increases exponentially toward the opposite end. Move the kink to the other side, and the stiffness profile flips. Move the kink to the middle, and the material becomes soft in the center and stiff toward both ends.
“The idea here is that we’ve essentially made an acoustic tractor beam that moves a kink and changes the way a material feels — while creating gradients of stiffness — on demand,” Boechler said.
Because the model material has no energy barriers, the researchers were able to use sound waves not just to move the kink, but do so predictably and step by step.
“We showed that if you send acoustic waves in from one side, they actually pull the kink toward where the sound came from,” Boechler said. “You can send a small pulse, and the kink moves a little. Send another pulse, and it moves a little more. It’s basically remote control for the material’s internal state.”
To demonstrate, the team built a life-sized experimental model consisting of a chain of stacked, rotating disks connected by springs. Each disk represents an individual atom, while the springs mimic atomic bonds. One disk — arranged differently from the rest — represents the kink. When short pulses of acoustic waves were sent into the structure, the kink was pulled toward the sound source, moving a few disks at a time. Each additional short burst of vibration nudged the kink a little farther. When longer vibrations were applied, the kink was continuously pulled across the entire length of the chain, effectively flipping which side of the chain was soft and which was stiff.
While the researchers can currently pull the kink rather than push it, they note that the level of control already surpasses anything achieved previously. The team also demonstrated that only certain sound frequencies cause the kink to move, while others have no effect.
Computer simulations also revealed that when a sound wave reaches the kink, part of the wave reflects and part passes through. Even so, the interaction transfers momentum to the kink and allows it to continue moving.
The system modeled in this study points to potential future applications such as materials with tunable stiffness, shape-changing structures and robust signal transmission.
“Right now, this is a toy model,” Boechler noted. “If something like this could be made into a real material, you could imagine structures that adapt on the fly — materials you can reprogram using sound.”
Next steps include exploring three-dimensional versions of the model system and investigating whether similar effects could exist at much smaller, even atomic, scales.
“This is fundamental research,” Boechler said. “But fundamental discoveries are often what end up advancing technology in the long run. Our work shows what becomes possible when you design materials with genuinely new properties.”
Full study: “Observation of mechanical kink control and generation via acoustic waves.” Additional study co-authors include Kai Qian (first author) and Nicolas Herard, UC San Diego; and Nan Cheng, Francesco Serafin and Kai Sun, University of Michigan.
This work was supported by the U.S. Army Research Office (grant W911NF-20-2-0182) and the U.S. Office of Naval Research (MURI N00014-20-1-2479).
END