Credit: Josu Irisarri.
When high-voltage electricity breaks out in open air, it rips through the atmosphere, forming chaotic, branching fingers of electric plasma. These electrical arcs are difficult to control. They branch out in seemingly random directions, following unpredictable paths dictated by subtle shifts in air density, electrical charge, and the pull of nearby conductive surfaces.
Now, an international team of scientists has discovered how to tame these erratic discharges using high-frequency sound. By projecting dynamic ultrasonic fields, researchers can trap the hot, low-density air generated by a spark and force it into precise, millimeter-accurate acoustic channels.
Because electric current naturally prefers to travel through this less dense air, the plasma dutifully follows the acoustic guide. The level of control is staggering. Researchers could bend high-voltage sparks around solid obstacles and direct them onto specific targets with millisecond response times.
This breakthrough could open the door to a strange new world of applications, from invisible wiring for high-voltage electronics to etching bacteria and creating midair haptic feedback you can feel on your bare skin.
Harnessing the Chaos of Plasma
Ultrasonic field guiding electric plasma. (A) Plasma spark without the ultrasonic field applied. (B) Plasma spark with the ultrasonic field. (C) Amplitude of the acoustic field (electrode in green). Scale bars, 1 cm. Credit: Science Advances.
Researchers at the Public University of Navarre, the University of Helsinki, and the University of Waterloo experimented with acoustic fields to see if sound could influence the electrical discharge.
“We observed this phenomenon more than one year ago, then it took us months to control it, and even longer to find an explanation,” says Dr. Asier Marzo from the Public University of Navarre, lead researcher of the work.
To achieve this, the team built a specialized testing arena. They placed a Tesla coil inside two concentric rings made up of 360-degree ultrasonic emitters. The Tesla coil generated an alternating current spark, which initially erupted into a messy, tree-like shape. But when they switched the ultrasonic rings on, the chaotic branches instantly collapsed into a single, focused line of plasma.
By electronically adjusting the strength and phase of the individual emitters, the researchers could tilt the focal point of the sound. As the sound moved, the spark obediently followed, locking onto specific electrodes in a grid.
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The Invisible Acoustic Funnel
So, how exactly does sound tell electricity where to go?
You might assume the sound waves are physically pushing the electrons, but the physics is much more elegant. The ultrasound is actually reshaping the atmosphere itself, creating an invisible, low-resistance tunnel for the electricity to travel through.
When a spark ignites, it violently heats the air immediately surrounding it, raising the temperature to roughly 70 degrees Celsius. Hot air naturally expands, which causes its density to drop. Because electricity always looks for the easiest path forward, it prefers to travel through this lower-density air, which offers a lower breakdown voltage.
This is where the ultrasound comes in. The high-frequency sound pulses exert an acoustic radiation force that actually traps and moves this hotter, less-dense air. The sound waves push the hot air into specific “antinodes”, which are areas of high acoustic amplitude.
The electrical plasma simply follows this artificial tunnel of hot air.
Sound Waves Over High-Powered Lasers
Long-exposure picture of the electric spark while the Tesla coil is translated inside an ultrasonic ring. (A) Side view while the coil is translated in one dimension. (B) top view while the coil is scanned in two dimensions using a CNC stage. In the right halves of the pictures, the simulated amplitude fields have been overlaid. Scale bars, 1 cm. Credit: Science Advances.
Before this discovery, the only reliable way to guide plasma in midair was to use lasers. These systems, known as Electrolasers, fire high-power optical pulses to heat the air and create a plasma channel for the spark to follow.
But using lasers has major drawbacks. They are expensive and require bulky, cumbersome equipment. They demand incredibly precise, microsecond timing to synchronize the laser pulse with the electrical discharge. Lasers can also introduce major safety risks, potentially blinding bystanders or damaging the materials they strike.
Ultrasound bypasses these hurdles entirely. The acoustic equipment is compact, affordable, and safe for human eyes and skin. Furthermore, ultrasonic fields can operate continuously without needing perfect synchronization with the electric spark.
“Precise control of sparks allows their utilization in a wide variety of applications, such as atmospheric sciences, biological procedures and selective powering of circuits,” comments Prof. Ari Salmi from the University of Helsinki.
The acoustic method is highly responsive, taking merely 15 to 35 milliseconds to grab a spark and stabilize its new path. Researchers can even intersect two acoustic focal points to create a curved field, allowing the plasma spark to gracefully bend around a physical obstacle, like a popcorn kernel, to reach its target.
Touching the Untouchable
One of the most surprising outcomes of this research is how it changes the rules of what electricity can hit.
Normally, a spark strongly prefers conductive metals. But with acoustic guidance, the researchers could coax the plasma to strike non-conductive materials like flat sheets of acrylic and paper at fixed, specific spots.
This unprecedented control paves the way for fascinating real-world uses. Engineers could use ultrasonically guided plasma to wirelessly switch high-voltage circuits without physical relays. It could be used to mill materials, weld with high precision, or precisely target colonies of bacteria in a petri dish.
There is one current limitation: the technique currently only works with alternating current (AC) sparks. When the team tried to guide direct current (DC) sparks, the attempt failed, likely because DC creates an “ionic wind” between the electrodes that disrupts the acoustic field.
Despite this, the potential for AC spark manipulation is vast, particularly in human-computer interaction. Because ultrasound can precisely target low-power plasma bolts directly onto human skin, it could create entirely new kinds of interfaces.
“I am excited about the possibility of using very faint sparks for creating controlled tactile stimuli in the hand, perhaps creating the first contactless Braille system,” says Josu Irisarri, first author of the publication from the Public University of Navarre.
The findings appeared in the journal Science Advances.