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Star Trek’s tractor beams, Princess Leia holograms and Tony Stark’s holographic interface. They are concepts from science fiction, but the Multi-Sensory Devices (MSD) group make them science fact
How acoustic levitation works
Instead of a single speaker, the MSD group uses arrays of them, up to 512 ultrasound speakers in fact. By turning these speakers on and off, they control the distribution of the waves with great precision.
They can create ‘pressure cages’ in mid-air, points with very little pressure in their centre but extreme pressure around them.
A small object placed in the ‘cage’ remains supported by the force of the surrounding waves, moving the ‘cage’ floats the object.
There’s no doubt that levitation can be a fun and magic-like experience, but there are many fascinating applications to explore.
Making the impossible possible
Professor Sri Subramanian, Dr Diego Martinez Plasencia and their team have been working on acoustic levitation for over a decade.
Other researchers had achieved levitation, but it was either in water or shown in one-off demonstrations. The scientific community deemed it impossible to dynamically achieve levitation from the type of low power commercial speakers the team used.
But this scepticism did not stop them trying, nor funders from supporting their blue-sky ambition.
The MSD group started by moving a single bead up and down. Then they manoeuvred the bead left and right.
At that stage, they had transducers (these convert energy from one form to another) at the top and bottom of the device. They wanted to have a single array at the bottom.
Sri said: “What makes this particularly interesting is that sound waves go forward and don’t come back. So you have a wave going forward and an object held by it being pulled towards you. This creates what science fiction calls a tractor beam.”
In collaboration with Professor Bruce Drinkwater of Bristol University, the team discovered the tractor beam effect in 2015 just before the 50th anniversary of the television series Star Trek. The press marked the anniversary by lauding the invention of the tractor beam as featured in Star Trek.
Next, Diego and Ryuji (Dr Ryuji Hirayama) focused on improving the performance of the algorithms that control the transducers.
They developed algorithms capable of updating the position of the levitated particles up to 40,000 times a second. This provided smoother motion, higher acceleration and speed.
By illuminating the bead and moving it quickly, it’s possible to trace an image in the air.
Think of drawing with a sparkler on Bonfire Night. These traces build up to create a holographic volumetric image.
Even though the device only uses inaudible ultrasound, the sampling rate of 40,000 frames per second enables the particle to emit sound we can hear.
Include the haptic effect gained from the vibrations of the ultrasound waves, and you have an all-singing, all-dancing particle.
Sri said: “Just a year before we did this, there had been a paper in [the science journal] Nature citing our work and saying it would never be possible to use ultrasound to create a 3D display.
We believed this ourselves until we tried. And it turned out that, in fact, we could create a multimodal 3D display, not only a visual display of light but also delivering audio and touch. All of it just from ultrasound.”
Applications through collaboration
The magical, gravity-defying quality lends itself to art and entertainment. A partnership with the Shanghai Academy of Art is exploring how artists can use acoustic levitation.
One installation combines it with traditional sculpture: a dragon opens its jaws to reveal a holographic fireball. Diego is also working with Lighthouse, the Brighton-based arts charity that connects art, technology, science and society.
Wonderland, a creative studio for the food and beverage industry, is using the levitation techniques. Park Row is its high-end DC Comics inspired restaurant in London, where patrons can sample a miniature floating gin and tonic.
The fact that acoustic levitation allows for materials to be manipulated without touch offers many opportunities. One such application is multi-material printing.
It’s expensive to 3D print in different materials because each material requires a specific nozzle. But acoustic levitation can add materials without using nozzles.
The technology could also volumetrically display what is being printed, so the user can visualise the final design and make changes mid-print. The MSD group are investigating these ideas with colleagues in UCL’s Mechanical Engineering department.
Because there are no handling and contamination issues, acoustic levitation is also interesting for life science researchers. Levitating a fruit fly could provide new insights into how an insect flies, for example.
The principles that trap and levitate objects in the air also apply inside the human body. Ultrasound is currently used to blast kidney stones.
Michael Bailey’s team at the University of Washington are working on clinical trials to test if acoustics can manoeuvre the fragments of stones left behind. These fragments could be relocated so the kidneys can dispel them naturally.
The team hope to move entire kidney stones using acoustic levitation. NASA, seeing the benefits for astronauts, is supporting this work.
Doctors use small cameras to examine the digestive tract. In the future, they could potentially use acoustic levitation to guide these cameras to particular areas.
In the same way, acoustics may also deliver small amounts of drugs precisely where they are needed. Microbubbles, spheres made of fatty molecules, could carry the drugs.
Diego Baresch at the University of Bordeaux is exploring this. With Valeria Garbin at Imperial College London, he showed, on a mock-up of human tissue, how acoustic tweezers can grab, move and burst microbubbles on demand.
Driven by curiosity
The MSD group take an experimental stance.
Sri explains: “You can work on a blue-sky project without knowing the possible applications. I never dreamt drug developers and life science researchers would be interested in our work. With curiosity-driven science, the results can be more than evolutionary; they can be revolutionary.”
Diego, an Associate Professor of Multisensory Interfaces, enthuses:
”All kinds of things may be possible. We are where TVs were in the 1920s. I find it very exciting to be working in the early stages of this technology.”
The group are keen to introduce acoustic levitation to a broad audience. They want people to think about what problems it could solve in their research or industry.
The pandemic temporarily put paid to the live exhibitions. But the team hopes to be exhibiting at New Scientist Live, the Royal Society Summer Science Festival and trade shows such as CES.
They want as many people as possible to witness acoustic levitation in real life. Seeing really is believing.