Study: Observation of mechanical kink control and generation via acoustic waves (DOI: 10.1038/s41467-026-68688-7)
An international research team including members from the University of Michigan, the University of California San Diego and Le Mans University has demonstrated that sound can remotely control how a material behaves.
The team showed for the first time that specific frequencies of acoustic waves can reliably move localized features in a material known as mechanical kinks. These kinks determine whether different regions of the material are soft or stiff.
"This opens the door to future technologies where you can remotely tune configurations and functionalities deep inside a material without cutting it open," said Xiaoming Mao, professor of physics and leader of the U-M cohort working on the study.
The research, which was published in the journal Nature Communications, was supported by federal funding from the U.S. Army Research Office and the U.S. Office of Naval Research. The Laboratory of Acoustics at Le Mans University also operates under the supervision of the French National Center for Scientific Research, or CNRS.
Kinks act as boundaries between two distinct internal states of a material. 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, but doing so has proven difficult. In most materials, kinks encounter energy barriers that pin them in place.
Although previous studies have shown it is possible to move kinks using sound waves, the resulting motion was typically chaotic and difficult to predict, said co-author Nicholas Boechler, associate professor of mechanical and aerospace engineering at UC San Diego. In the new study, the team used theoretical, computer and physical models to show that sound can control kinks in a controllable manner.
"You can send a small pulse, and the kink moves a little. Send another pulse, and it moves a little more. It's basically a remote control for the material's internal state," Boechler said. "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."
Based on the team's theoretical and computational work, the researchers showed that a key feature is that moving the material's mechanical kink must not cost energy. It's a rare feature, but one that's achievable in what are known as metamaterials, which are engineered materials whose behaviors are dictated more by their structure than their composition. In this modeled material without energy barriers, the researchers were able to use sound waves not just to move the kink, but do so predictably and step by step.
The team also built a life-sized experimental model to demonstrate. The physical model consisted of a chain of stacked, rotating disks connected by springs, where each disk represents an atom and 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.
"Right now, this is a toy model," Boechler said. "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."
The project's U-M contingent also included Kai Sun, professor of physics, along with Nan Cheng and Francesco Serafin, who served as a doctoral student researcher and a postdoctoral researcher, respectively. The U-M group contributed to the theory behind kink behavior in the model system and are continuing to explore that phenomenon in more disordered metamaterials.
Georgios Theocharis, a CNRS scientist, was the team lead at the Laboratory of Acoustics at Le Mans University.
