Scientists Levitate Particles With Sound to Defy Gravity

The goal of the research is to understand how materials cluster together in the absence of gravity.

In what may be more magic trick than science, scientists have successfully used sound waves to levitate and study particles. The goal of the research is to understand how materials cluster together in the absence of gravity.

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Assembling particles

"Much of the universe is made up of particles assembling," said Heinrich Jaeger, the Sewell Avery Distinguished Service Professor of Physics. 

"With acoustic levitation, we have a beautiful model system to study assembly at scales visible to the human eye, where we can track each particle with precision, and then relate the results to a wide range of often much more microscopic phenomena."

What the researchers were examining are the laws governing the interactions of particles. In this experiment in particular, Jaeger and his team were interested in the shape of prototypical clusters.

They were studying the forms these clusters take when starting from a single particle, more are added one by one. To achieve this, they used sound waves to levitate plastic particles.

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They then analyzed how these particles interacted with each other focusing on how they assembled and reassembled into different configurations.

They also used high-speed cameras to track the levitated particles. What they found was that five particles or fewer clustered densely in only one configuration.

However, at six particles and more, the particles got creative making several different shapes. The team called the three shapes created by six particles "parallelogram, chevron and triangle."

At seven particles, four shapes were made. These were called a "flower, a turtle, a tree or a boat."

"Six particles is the minimum needed to change between different shapes, which is where things get interesting," said co-first author Anton Souslov, then a UChicago postdoctoral researcher and now on the faculty at the University of Bath.

"For us scientists, defying gravity to levitate dust also has this more fundamental interest of developing Earth-based experiments to understand how bodies in space like planets start to form."

An unexpected outcome

One unexpected outcome of the study was discovering that the sound frequency could influence the cluster shapes.

"A surprise was that by changing the sound wave frequency, we could manipulate the clusters and influence the shape that emerged," said graduate student Melody Lim, the first author on the paper.

Now, the researchers are looking into modeling the physics behind such acoustic forces. The goal is to control the assembly process hopefully.

The team is also exploring how acoustic levitation impacts even larger numbers of particles. This will logically result in more complex structures.

The new study is published in Nature Physics

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