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Sue Shi and Scott Waitukaitis overcame “acoustic collapse,” a fundamental limitation of acoustic levitation, by adding another force: electrical charge.
Who has never dreamed of countering gravity and making objects hover above the ground? A team of Austrian physicists has now overcome a fundamental limitation of acoustic levitation, “violated” one of Newton’s laws, and used sound to make multiple objects “float” in the air.
Using sound to make objects float usually works well when levitating just a particlebut when several enter the scene, they end up collapsing and forming a cluster in mid-air.
A team of physicists from the Austrian Institute of Science and Technology (ISTA) has now found a way to keep them at bay, using electrical charge.
The results of their study, presented in an article published last week in the journal PNASmay have applications in materials sciencerobotics and microengineering.
Em 2013, Scott Waitukaitiscurrently an assistant professor at ISTA, became interested in the use of acoustic levitation as a tool to study various physical phenomena. At the time, only a handful of research groups used this technique for similar purposes.
“Although acoustic levitation was used in acoustic holograms and on volumetric displays, the approaches were very application-oriented. I had the feeling that the technique could serve much more fundamental purposes”, remember Waitukaitis whether I do ISTA.
After creating his research group at ISTA, Waitukaitis began to set up several experiments based in controlling matter through sound.
Overcoming “acoustic breakdown”
One of the curious aspects of acoustic levitation is that the method works well when only one particle is levitated. However, if several particles are introduced, they suddenly come together like magnets, in mid-air.
It is “acoustic collapse” It occurs because the dispersion of sound in particles creates attractive forces between them. It is a central limitation of the technique.
“Originally, we were trying to find a way to separate the levitated particles to form crystals — specific repetitive patterns”, explains Sue Shidoctoral student in the Waitukaitis group and first author of the study.
Only later did they realize that solve the collapse problemkeeping the particles separated, was even more important.
The key was in add another force that counteracts the collapse; and then behold electric charge and electrostatic repulsion come into play. “By counterbalancing the sound with electrostatic repulsion, we were able to keep the particles away from each other”, summarizes Shi.
After developing a method to charge the particles, the team found that I could adjust this load in order to levitate them in various configurations — including systems in which all particles remained separate, others in which they completely collapsed and even “hybrids”, with components both separate and collapsed.
Scientists have also managed make particles “bounce” on the loaded lower reflector plate of the levitation device, thus switching between different configurations.
In collaboration with Carl Goodrichassistant professor at ISTA, and with the doctoral student Maximilian Huebl, the team developed simulations to explain all observed configurations, based on the balance between sound scattering forces and electrostatic forces.
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Particle to be levitated in the acoustic levitation system of the Waitukaitis group laboratory at ISTA
“Violate” Newton’s law
As so often happens in science, some of the phenomena that the team did not predictturned out to be even more interesting. Some of the complex behaviors observed pointed to the presence of “non-reciprocal” interactions — situations that seem “violate” Newton’s 3rd law.
Among the most striking cases were certain arrangements of particles that began to rotate spontaneously, or pairs of particles that seemed to be chasing each other.
Strictly speaking, Newton’s third law is not violated — the amount of extra motion gained by the particles is lost to the sound.
Previous theoretical work had already predicted that effects of this type should exist in acoustic levitation systems, but had not been observedprecisely because the particles always collapsed into a single cluster.
“You cannot study how individual particles interact if you cannot keep separate“, underlines Waitukaitis. “By introducing electrostatic repulsion, we are now able to maintain stable and well-separated structures. This finally gives us a controllable platforml to investigate these subtle, non-reciprocal effects.”
The method developed by the team opens up new possibilities for manipulating matter in mid-air, with potential applications in materials sciencemicro-robotics and other areas that depend on the formation of controlled structures and dynamics from small building blocks.
The team is already using this approach to study the non-reciprocal effects that it now has access to.
“At first it was frustrating seeing these hybrid configurations and strange rotations and dynamics — kept me from getting the clean, stable crystalline structures I was looking for,” admits Shi.
But presenting her results at conferences and seeing the enthusiasm of other scientists helped her see these phenomena as fascinating in their own right. “That’s the curious part of experiments: many of the most interesting discoveries come precisely from things that don’t go as planned”.
Science sometimes even has a touch of magic.
