New experimental study tackles the unsolved mystery of ‘nanobubbles’

Nanobubbles are extremely small (i.e., nanoscopic) gaseous cavities that some physicists have observed in aqueous solutions, usually after specific substances have been dissolved in them. While some studies have reported observing these incredibly tiny bubbles, some scientists have argued that they were just solid or oily residue formed in experiments.

Researchers from the Centro de Investigación y de Estudios Avanzados Unidad Monterrey and the Centro de Investigación en Matemáticas Unidad Monterrey in Mexico recently conducted an experiment to further investigate the nature of these elusive and mysterious objects, particularly when xenon and krypton were dissolved in water. Their study, presented in Physical examination lettersidentified the formation of what the team calls “nanoblobs”, but found no evidence of nanobubbles.

“Our goal was to create xenon and krypton nanobubbles using a clean method,” Carlos Ruiz Suarez, one of the researchers who conducted the study, told Phys.org. “I must say that many scientists argue that nanobubbles, despite being used in many applications, do not exist. Rather, they are believed to be oily/solid contaminants formed during experiments.”

To solve the “mystery” of nanobubbles, Ruiz Suarez and his colleagues devised a “clean” method that theoretically should have allowed them to produce “real” nanobubbles. This method involved dissolving the two noble gases xenon and krypton in water, applying high pressure to them, then depressurizing and inspecting the resulting liquid.

The team evaluated the results of this procedure in molecular dynamics simulations (MDS) and laboratory experiments. While they did observe nanobubble-like particles, when they analyzed these particles, they were surprised to find that they were most likely gas-water amorphous structures, rather than gas bubbles.

“To gather the noble atoms to be nucleated in bubbles, we had to increase their concentrations in the aqueous medium”, explained Ruiz Suarez. “By performing MDS, we found that the correct proportions between water molecules and noble atoms were around 30 water molecules/atom. So, we had to build a high-pressure cell to force the atoms to dissolve in water pushing the gas inside.”

Xenon and krypton are two hydrophobic gases. This means that they can only enter water and aqueous solutions under high pressures (over 360 bar or atmospheres). Once they enter water, however, they can bond to each other through hydrophobic and van der Waals forces.

“There is currently no way to see inside the cell, but we assumed the bubbles existed because we believed our MDS,” Ruiz Suarez said. “The next step in our work was to depressurize the sample and see the bubbles. However, to our surprise, there were no bubbles, but something else: nanostructures formed of gas and water, which we called nanoblobs. These are sui generis structures that give rise to hydrated clathrates.”

The existence of nanobubbles remains a hotly debated topic in particle physics, and recent work by these researchers may help solve this mystery. Much like xenon and krypton, many other gases used to form nanobubbles can also form clathrate hydrates (i.e. water structures with molecules inside). Overall, the team’s findings therefore suggest that what many previous studies have identified as “nanobubbles” may instead be these amorphous nanostructures formed by clathrate hydrates.

“It’s important to notice that when an existing physical theory can’t explain experimental results, physicists like to call it a disaster,” Ruiz Suarez said. “Since nanobubbles have a high pressure inside (the smaller they are, the higher the pressure), the theory says that their lifetime is very short (on the order of microseconds). However, observations revealed that they have been around for much longer, so it has been called the Laplace pressure bubble disaster.”

If the results collected by this team of researchers are valid and reliable, they could greatly contribute to the current understanding of nanobubbles. Essentially, their findings suggest that Laplace’s pressure bubble catastrophe does not exist, because the previously observed “nanobubbles” are instead “nanoblobs”, or alternative structures resulting from clathrate hydrates in gases used experimentally.

“We are currently building an experimental device that will allow us to see inside the cell and observe the nanobubbles at high pressure,” said Ruiz Suarez. “We would like to see how they evolve when we lower the pressure and when they become clathrate hydrates. Alongside this, we are also studying other important gases like oxygen and carbon dioxide.”

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