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Sound waves could power building of new materials

24-11-2020
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Ever spoken something into existence? We’re not quite at that stage yet, but using sound waves could be the next evolution in manufacturing.

Australian researchers have revealed how high-frequency sound waves can be used to build new smart materials, make smart nanoparticles and even deliver drugs to the lungs for painless, needle-free vaccinations.

While sound waves have been part of science and medicine for decades – such as ultrasounds – the technologies have always relied on low frequencies. 

The boffins at RMIT University have shown how using high frequencies could revolutionise the field of ultrasound-driven chemistry.

The team, headed by lead researcher Distinguished Professor Leslie Yeo, generates high-frequency sound waves on a microchip to precisely manipulate fluids or materials.

They used the sound waves to drive crystallisation for the sustainable production of metal-organic frameworks, or MOFs.

Predicted to be the defining material of the 21st century, MOFs are ideal for sensing and trapping substances at minute concentrations, to purify water or air, and can also hold large amounts of energy, for making better batteries and energy storage devices.

“When we couple high-frequency sound waves into fluids, materials and cells, the effects are extraordinary,” Yeo says.

“We’ve harnessed the power of these sound waves to develop innovative biomedical technologies and to synthesise advanced materials.”

While the conventional process for making a MOF can take hours or days and requires the use of harsh solvents or intensive energy processes, the RMIT team has developed a clean, sound wave-driven technique that can produce a customised MOF in minutes and can be easily scaled up for efficient mass production.

Sound waves can also be used for nano-manufacturing 2D materials, which are used in myriad applications from flexible electric circuits to solar cells.

“Our discoveries have also changed our fundamental understanding of ultrasound-driven chemistry – and revealed how little we really know,” Yeo adds.

“Trying to explain the science of what we see and then applying that to solve practical problems is a big and exciting challenge.”

Ultrasound has long been used at low frequencies – around 10 kHz to 3 MHz – to drive chemical reactions, a field known as “sonochemistry”. At these low frequencies, sonochemical reactions are driven by the violent implosion of air bubbles.

But it turns out that if you up the frequency, these reactions change completely. The researchers saw behaviour that had never been observed with low-frequency ultrasound.

“We’ve seen self-ordering molecules that seem to orient themselves in the crystal along the direction of the sound waves,” Yeo says.

“The sound wavelengths involved can be over 100,000 times larger than an individual molecule, so it’s incredibly puzzling how something so tiny can be precisely manipulated with something so big.

“It’s like driving a truck through a random scattering of Lego bricks, then finding those pieces stack nicely on top of each other – it shouldn’t happen!”

The researchers also detail various exciting applications of their pioneering work, including delivery of drugs to the lungs through inhalation, creating of drug-protecting nanoparticles, nano-manufacturing 2D materials and breakthrough smart materials 

Professor Yeo and his team have spent over a decade researching the interaction of sound waves at frequencies above 10 MHz with different materials, but they are only now starting to understand the range of strange phenomena they often observe.

The next steps for the RMIT team is on scaling up the technology. 

At a low cost of just $US0.70 per device, the sound wave-generating microchips can be produced using the standard processes for mass fabrication of silicon chips for computers.

“This opens the possibility of producing industrial quantities of materials with these sound waves through massive parallelisation – using thousands of our chips simultaneously,” Yeo said.

The team at the Micro/Nanophysics Research Laboratory, in RMIT’s School of Engineering, is one of just a few research groups in the world bringing together high-frequency sound waves, microfluidics and materials. 

Yeo says the research challenges long-held physics theories, opening up a new field of “high frequency excitation” in parallel to sonochemistry.

“The classical theories established since the mid-1800s don’t always explain the strange and sometimes contradictory behaviour we see – we’re pushing the boundaries of our understanding.”

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