New Method Developed to Remotely Control Material Behavior Using Sound

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) has demonstrated a new method to remotely control how a material behaves using sound.

Potential Applications

This discovery could lead to the development of protective gear, robotic muscles, or medical implants that can adjust their stiffness on demand.

Mechanism of Control

Published in Nature Communications, the study revealed for the first time that specific frequencies of acoustic waves can reliably move localized features known as mechanical kinks. These kinks determine whether different regions of the material are soft or stiff.

Kinks represent boundaries between two distinct internal states of a material, where the building blocks are oriented differently. They mark where a material deforms, such as in permanently bent metals or separating DNA strands.

Controlling kinks can reshape a material's behavior, altering its soft versus stiff regions. However, in most materials, kinks are pinned in place by energy barriers, and previous attempts to move them with sound resulted in chaotic motion.

Novel Material Design

To overcome these challenges, the research team, including Nicholas Boechler (UC San Diego), Xiaoming Mao (University of Michigan), and Georgios Theocharis (CNRS), modeled a material where moving the kink costs no energy. This unusual property was achieved by designing the material based on its structure rather than its composition.

In this model material, the region where the kink is located is soft, while the rest of the material becomes progressively stiffer. Moving the kink repositions this soft region. For instance, if the kink is moved to one end, that end becomes soft, and stiffness increases exponentially toward the opposite end. If moved to the center, the material becomes soft in the center and stiff toward both ends.

Remote Control Through Sound

Because the model material lacks energy barriers, researchers were able to use sound waves to move the kink predictably and step by step. Acoustic waves sent from one side pull the kink toward the sound source.

A small pulse moves the kink slightly, and subsequent pulses advance it further, demonstrating remote control over the material's internal state. Only certain sound frequencies activate this movement.

Experimental Demonstration

The team built a life-sized experimental model: a chain of stacked, rotating disks connected by springs, representing atoms and bonds. One distinct disk represented the kink.

Short acoustic wave pulses pulled the kink a few disks at a time. Longer vibrations continuously pulled the kink across the entire chain, effectively flipping the stiffness profile of the material.

Computer simulations further confirmed that when a sound wave reaches the kink, it transfers momentum, allowing the kink to move despite partial reflection and transmission of the wave.

Future Outlook

While currently a model system, this research suggests potential for future applications in materials with tunable stiffness, shape-changing structures, and robust signal transmission.

The next steps involve exploring three-dimensional versions of the system and investigating similar effects at atomic scales.