An acoustic tractor beam that makes materials soft on one side and stiff on the other

Take a material and make one end soft enough to cushion an impact. Now, without touching it, flip the whole thing so the other end goes soft instead. That is the promise behind a new experiment from researchers at UC San Diego, the University of Michigan, and France's CNRS at Le Mans University - and they pulled it off using nothing but sound.

Before sound: the kink problem

The trick hinges on structures called mechanical kinks. A kink is a boundary inside a material where the internal building blocks switch orientation. On one side of the kink, the material might feel pliant. On the other, it stiffens dramatically. Materials scientists have understood kinks for decades - they show up wherever metals permanently bend, wherever DNA strands separate - but controlling them has been another matter entirely.

In most materials, kinks get pinned in place by energy barriers. Previous attempts to shove them around with acoustic waves produced chaotic, unpredictable motion. The kink would lurch, stall, or jump past its target. Not useful if you want precision.

After sound: step-by-step remote control

The team, led by UC San Diego mechanical engineering professor Nicholas Boechler alongside co-leads Xiaoming Mao (Michigan) and Georgios Theocharis (CNRS), approached the problem by eliminating the energy barriers altogether. They designed a model material whose behavior depends on its geometric structure rather than its chemical composition. In this system, sliding the kink from one position to another costs zero energy - a rare and deliberately engineered property.

With the barriers gone, sound waves could move the kink cleanly. And the direction was counterintuitive: the kink gets pulled toward the sound source, not pushed away from it. Send a short acoustic pulse from the left, and the kink inches leftward. Send another, it moves a bit more. Boechler described it as "remote control for the material's internal state."

The stiffness consequences are immediate. Wherever the kink sits, that region goes soft. The rest of the material grows progressively stiffer with distance from the kink - exponentially so. Push the kink to one end, and that end becomes compliant while the opposite end locks rigid. Center the kink, and the material goes soft in the middle with stiffness climbing toward both edges.

A chain of spinning disks

To prove the concept experimentally, the researchers built a physical prototype: a chain of stacked, rotating disks connected by springs. Each disk represents an atom; the springs mimic atomic bonds. One disk, oriented differently from its neighbors, serves as the kink.

Short acoustic pulses sent into the chain pulled the kink toward the source, a few disks at a time. Longer, sustained vibrations dragged it across the entire structure in one continuous sweep, flipping which side was soft and which was stiff. The team also showed that only certain frequencies triggered kink motion - others passed through without effect.

Computer simulations revealed the underlying mechanics. When a sound wave hits the kink, part of it reflects and part transmits through. But even with that split, enough momentum transfers to keep the kink moving in a predictable direction. The researchers can currently only pull the kink, not push it, but even that one-directional control far surpasses anything demonstrated before.

What tunable stiffness could look like

The applications, if the concept scales, are tantalizing. Protective gear that stiffens on impact and softens afterward. Robotic muscles that change compliance without mechanical actuators. Medical implants that adjust their flexibility to match healing tissue. Shape-changing structures that reconfigure without motors or hinges.

But the emphasis falls heavily on "if." The current experiment uses a macroscopic chain of disks - a tabletop demonstration, not a deployable material. Translating zero-energy-barrier kink physics into real-world solids at practical scales remains an open engineering challenge.

A toy model with real physics

Boechler was candid about the gap between demonstration and application. "Right now, this is a toy model," he said. The next steps involve exploring three-dimensional versions of the system and investigating whether similar zero-barrier kink behavior can exist at much smaller scales - potentially down to atomic dimensions.

The work also carries a limitation common to proof-of-concept studies in metamaterials: the designed structure achieves its properties through careful geometric arrangement, which means manufacturing complexity could be significant. And the one-directional control - pulling but not pushing - means full bidirectional reprogramming would require sound sources on both sides of the material, or a different approach entirely.

Still, what the experiment establishes is that controllable, stepwise kink motion via acoustics is physically possible. That is a genuinely new result. Prior work showed kinks could be kicked around by sound, but never walked from point A to point B with this kind of precision.

The study was published in Nature Communications. First author Kai Qian and co-authors Nicolas Herard (UC San Diego), Nan Cheng, Francesco Serafin, and Kai Sun (University of Michigan) contributed to the work. Funding came from the U.S. Army Research Office (grant W911NF-20-2-0182) and the U.S. Office of Naval Research (MURI N00014-20-1-2479).