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Acoustic levitation

Home acoustic levitation device unveiled by researchers

Image credit: University of Bristol

Researchers at the University of Bristol have developed a device to levitate objects safely which can be constructed at home with off-the-shelf components. The acoustic levitator, TinyLev, is capable of levitating small objects such as ants, blobs of ketchup and droplets of blood.

In 2000, future Nobel Laureates Andre Geim and Michael Berry collected the Ig Nobel Prize in Physics for the magnetic levitation of a live frog. Magnetic levitation uses a powerful magnetic field to suspend objects, from high-speed trains to products displayed in a shop window.

Levitation, beyond proving an amusing parlour trick for physicists, has important applications across sectors such as transport and medicine. Levitating a droplet of blood in a diagnostic test, for instance, produces better results than testing on a microscope slide, which interferes with test results.

While considerably less strong than magnetic levitation, objects can also be suspended using sound waves. Acoustic levitation uses the momentum carried by powerful sound waves to push objects from all directions, effectively trapping them in mid-air. Harnessing the ultrasound waves – which are too high-pitched to be heard by humans – makes it possible to levitate small objects without producing deafening noise in the process.

“Acoustic levitation supports a wide range of materials, from liquids to living things,” said Dr Asier Marzo, the University of Bristol researcher behind the TinyLev told E&T. “[It is] one of the safest methods for holding substances in mid-air; there are no crazy-powerful magnets that could crush your hand, or lasers that would make you blind.”

Currently, acoustic levitators are made using Langevin horns: devices which produce sound from curved surfaces, which focus the sound waves. Langevin horns must be custom-made from metal, require high voltages to run and, most significantly, must be carefully tuned to a specific frequency. Small changes in temperature can cause detuning.

TinyLev, based on the work of Dr Marzo and his colleagues in ultrasonic devices. It uses off-the-shelf components, does not require dangerous high voltages, does not require tuning and can operate for long periods of time.

The levitator is made up of two arrays of low-voltage ultrasonic transducers arranged above and below the object in a 3D-printed frame. The positions and orientations of the transducers are carefully designed to achieve an acoustic focus when switched on. The transducers are packed closely to provide a strong vertical force: levitation against gravity. Smaller horizontal forces help stabilise the object.

Asier Marzo © 2017

Experimenting with TinyLev, Dr Marzo and his colleagues successfully levitated an ant, droplets of condiments, water and electronic components. TinyLev can be used on objects up to 4mm in diameter – half a wavelength of the ultrasound waves providing the force – and up to a density of 2.2g/cm3 (approximately as dense as concrete). Objects must also be a regular shape.

“If an object is irregular – like a tadpole with the tail extended – it will become unstable and start to spin until being ejected from the acoustic trap,” explained Dr Marzo. “One solution is to place the irregular sample inside a droplet of water.”

While TinyLev may not be able to recreate the prizewinning frog levitation performed by Geim and Berry – even with a tadpole – its unique properties, such as its ability to run for long periods of time without detuning, could enable experiments that were not previously possible.

“Acoustic levitation has been employed for studying living things in microgravity, mixing chemical components without any recipient or analysing samples free from reflections of the microscope slide,” Dr Marzo told E&T.

“The applications [of TinyLev] are still the same, but now they will be accessible to every research lab […] the capability of our levitator to work for extensive periods of time enabling novel long-term experiments like germinating a levitating seed or studying the evolution or a zebrafish from embryo to adult.”

“These experiments could provide insights on the viability of cultivating plants or human reproduction in zero-gravity.”

Dr Marzo’s University of Bristol team had previously released a simple acoustic tractor beam which allows non-researchers to create a piece of science-fiction technology at home. Continuing to bring lab equipment to the interested masses, the researchers have now developed an “instruction pack” for people who want to assemble an acoustic levitator at home.

An amateur can assemble their own TinyLev using a 3D-printed frame, an Arduino (a basic microcontroller) a motor diver and parking sensors. The idea is that researchers and even school students can construct a device on their own.

“Since our levitator is below $80, easy to assemble and uses non-dangerous voltage, we also expect schools and DIY-enthusiasts to build it,” said Dr Marzo. “Perhaps it could become a common science demonstration, like a slinky.”

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