Discovery solves century-old mystery behind geological phenomenon

Institute for Frontier Materials researchers show how pressure solution can be used for nanofabrication on glass by selectively etching the glass underneath a pattern.

IFM researchers have shown for the first time how pressure solution can be used for nanofabrication on glass by selectively etching the glass underneath a pattern. This etching is done in pure water without the need for any chemicals or expensive equipment. PhD student Kilian Fraysse, who made the discovery recently inscribed the world’s smallest Australian flag into glass – measuring about two microns by four microns – half the width of the average human hair.

IFM researchers have solved a 100-year-old mystery of the mechanism behind an important geological phenomenon – and it has unlocked the potential to transform the mining industry and the development of next-generation nanomaterials.

Although ‘pressure solution’, which takes place during the formation of sedimentary rocks such as sandstone, was first observed by geologists in 1908, it has never been properly understood until now.

New research, recently published in the Journal of the American Chemical Society, shows that the  reaction is fundamentally an electro-chemical reaction – which has allowed researchers understand how to control the dissolution reaction using pure water and a common tool that can be found at any research facility. This new approach has the potential to save time, money and resources within the mining industry, and improve the impact on the environment by avoiding toxic chemicals.

“The applications for being able to control the dissolution reaction are potentially too many to really pin down – it can really apply to anywhere you would want to something quickly, for instance in the processing of minerals,” Dr Wren Greene said.

“For example, to dissolve silica at the same rate as we dissolved it in our experiments, traditionally you would need to use a 50 per cent solution of hydrofluoric acid, which is probably one of the most dangerous and aggressive acids. 

“This acid is used throughout the semiconductor industry for etching silicon wafers and for doing wet etching processes with.

“Our experiments showed that we can basically achieve those same etching rates in pure water. 

“That’s where the innovation really comes from – it’s in your ability to control that reaction, to accelerate dissolution reaction and to basically do it in a way that doesn’t involve nasty chemicals and acids that are currently use now.”

IFM researchers demonstrate how pressure solution can be used to selectively etching under a pattern, which can be used for nanofabrication.
IFM researchers demonstrate how pressure solution can be used to selectively etch under a pattern.

Using pure water and an atomic force microscopy (AFM) tip the researchers used this solution enhancement effect to rapidly remove any inorganic matter, such as silica, gallium nitride or silicon.

The only other technology available that uses a similar process is the NanoFrazor, which uses a laser heated AFM tip to burn a polymer away. The NanoFrazor system is extremely expensive and hard to come by, with only one system found in Australia, which currently costs about $1000p/h to use.

However, this new technique is not only easily accessible and affordable, but more functional and, with the right software, has the potential to turn any AFM into a Nanofabrication tool like the Nanofrazer.

“We discovered that just by pressing a second surface against the dissolving surface and vibrating the pressure, we are able to speed up the dissolution of silica by more than 600,000 times,” Dr Greene says.

“Because the pressure solution effect only occurs where the two materials contact each other, the effect is very localised and can be used to create nanomaterials, without expending a lot of energy or using harmful chemicals.”

PhD student Kilian Fraysse, who made the discovery, said the rate enhancing mechanism depended upon the effect that the pressure oscillations had on the electric fields at the silica surface, which regulate how quickly dissolved silica molecules can be transported away from the surface. 

“The second surface combined with the pressure effectively catalyses the dissolution of the silica,” Mr Fraysse said.

“While geologists always imagined pressure solution to be a rather slow process, we show that under the right conditions, it can happen very, very quickly.”

Dr Greene says this new way of understanding pressure solution also has the potential to be used on other types of etching work, such as chemical mechanical polishing.

“I envision processes like this being used in order to condition ores for processing in a way that doesn’t use acids or caustic bases, and potentially be more environmentally friendly,” he says.

“But there’s a huge engineering leap that needs to be made and we have got to learn how to harness the effect.”