Sound Waves Direct Particles to Self-Assemble, Self-Heal

By Sarah Yang

An elegantly simple experiment with floating particles self-assembling in response to sound waves has provided a new framework for studying how seemingly lifelike behaviors emerge in response to external forces.

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) demonstrated how particles, floating on top of a glycerin-water solution, synchronize in response to acoustic waves blasted from a computer speaker.

The study, published today (Monday, June 19) in the journal Nature Materials, could help address fundamental questions about energy dissipation and how it allows living and nonliving systems to adapt to their environment when they are out of thermodynamic equilibrium.

Close up photograph of the self-assembling particles in the clear acrylic tube. These particles consist of cut plastic straws (blue) sealed to a flat plastic chip (black), which float on top of a water-glycerin solution. (Credit: Chad Ropp/Berkeley Lab)

“Dynamic self-assembly under non-equilibrium is not only important in physics, but also in our living world,” said Xiang Zhang, corresponding author of the paper and a senior faculty scientist at Berkeley Lab’s Materials Sciences Division with a joint appointment at UC Berkeley. “However, the underlying principles governing this are only partially understood. This work provides a simple yet elegant platform to study and understand such phenomena.”

To hear some physicists describe it, this state of non-equilibrium, characterized by the ability to constantly change and evolve, is the essence of life. It applies to biological systems, from cells to ecosystems, as well as to certain nonbiological systems, such as weather or climate patterns. Studying non-equilibrium systems gets theorists a bit closer to understanding how life – particularly intelligent life – emerges.

However, non-equilibrium systems are complicated and hard to study because they are open systems, Zhang said. He noted that physicists like to study things that are stable and in closed systems.

Transient response of dynamic self-assembly. Top portion shows the position of the particles (blue) while they self-assemble in response to sound that is incident from the left (red arrow). Bottom portion shows the time response of the transmission spectrum of the system (blue), which is compared to theoretical spectrum (black). The red line denotes the wavelength of the monotone input sound. (Credit: Chad Ropp/Berkeley Lab)

“We show that individually ‘dumb’ particles can self-organize far from equilibrium by dissipating energy and emerge with a collective trait that is dynamically adaptive to and reflective of their environment,” said study co-lead author Chad Ropp, a postdoctoral researcher in Zhang’s group. “In this case, the particles followed the ‘beat’ of a sound wave generated from a computer speaker.”

Notably, after the researchers intentionally broke up the particle party, the pieces would reassemble, showing a capacity to self-heal.

Ropp noted that this work could eventually lead to a wide variety of “smart” applications, such as adaptive camouflage that responds to sound and light waves, or blank-slate materials whose properties are written on demand by externally controlled drives.

While previous studies have shown that particles are capable of self-assembly in response to an external force, this paper presents a general framework that researchers can use to study the mechanisms of adaptation in non-equilibrium systems.

“The distinction in our work is that we can predict what happens – how the particles will behave – which is unexpected,” said another co-lead author Nicolas Bachelard, who is also a postdoctoral researcher in Zhang’s group.

As the sound waves traveled at a frequency of 4 kilohertz, the scattering particles moved along at about 1 centimeter per minute. Within 10 minutes, the collective pattern of the particles emerged, where the distance between the particles was surprisingly non-uniform. The researchers found that the self-assembled particles exhibited a phononic bandgap – a frequency range in which acoustic waves cannot pass – whose edge was inextricably linked, or “enslaved,” to the 4 kHz input…

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This article (Sound Powered Self-Assembling and Self-Healing Particles) was originally published on Berkeley Lab and syndicated by The Event Chronicle. Found via Energy Fanatics.

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