It is possible that the smallest bubble ever photographed has been caught on camera by scientists at Northeastern University in Illinois, US.

In a world first, researchers have witnessed hydrogen and oxygen atoms merge to form tiny, nano-sized bubbles of water. The event, captured in real-time and at a molecular scale, happened during a study where scientists sought to understand how palladium, a rare metallic element, catalyses the gaseous reaction to generate water.

By witnessing the reaction at the nanoscale, the Northwestern team unravelled how the process occurs and even uncovered new strategies to accelerate it. As the reaction does not require extreme conditions, the researchers say it could be harnessed as a practical solution for rapidly generating water in arid environments, including on other planets.

Vinayak Dravid, senior author of the study, said, “By directly visualising nanoscale water generation, we were able to identify the optimal conditions for rapid water generation under ambient conditions. These findings have significant implications for practical applications, such as enabling rapid water generation in deep space environments using gases and metal catalysts, without requiring extreme reaction conditions.

Dravid draws comparisons with the movie The Martian where Matt Damon’s character Mark Watney burns rocket fuel to extract hydrogen and then adds oxygen from his oxygenator. Our process is comparable, except that the Northwestern team has bypassed the need for fire and other extreme conditions by simply mixing palladium and gases together.

Mystery unravelled

Since the early 1900s, researchers have known that palladium can act as a catalyst to rapidly produce water but how exactly this reaction occurs has remained a mystery.

Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802. Part of the platinum group of elements, palladium has the lowest melting point and is the least dense of the group.

More than half the supply of palladium is used in catalytic converters in car exhausts to convert harmful gases into nontoxic substances. It is also used in electronics, dentistry, medicine, hydrogen purification, chemical applications, groundwater treatment and jewellery.

It is also a key component of fuel cells, in which hydrogen and oxygen react to produce electricity, heat and water.

Yukin Liu, the first author of the study, said, “[Palladium producing water] is a known phenomenon, but it was never fully understood, because you needed to be able to combine the direct visualisation of water generation, and the structure analysis at the atomic scale, to figure out what is happening with the reaction and how to optimise it.”

However, viewing the process with atomic precision was simply impossible until nine months ago. In January 2024, Dravid’s team unveiled a method to analyse gas molecules in real-time.

The researchers developed an ultra-thin glassy membrane that holds gas molecules within honeycomb-shaped nanoreactors, so they can be viewed within electron microscopes.

With the new technology, researchers can examine samples in atmospheric pressure gas at a resolution of just 0.102 nanometers, compared to a 0.236-nanometer resolution using other specialised tools. The technique also enabled, for the first time, concurrent spectral and reciprocal information analysis.

Kunmo Koo, first author of the research paper, said, “Using the ultrathin membrane, we are getting more information from the sample itself as information from the thick container interferes with the analysis.”

Smallest bubble ever seen

Using the new technology, Dravid, Liu and, Koo examined the palladium reaction. First, they saw the hydrogen atoms enter the palladium, expanding its square lattice. But when they saw tiny water bubbles form at the palladium surface, the researchers could not believe their eyes.

“We think it might be the smallest bubble ever formed that has been viewed directly,” Liu said. “It’s not what we were expecting. Luckily, we were recording it, so we could prove to other people that we weren’t crazy.”

“We were sceptical,” Koo added. “We needed to investigate it further to prove that it was water that formed.”

The team implemented a technique, called electron energy loss spectroscopy, to analyse the bubbles. By examining the energy loss of scattered electrons, researchers identified oxygen-bonding characteristics unique to water, confirming the bubbles were, indeed, water. The researchers then cross-checked this result by heating the bubble to evaluate the boiling point.

It is the same technique that was used to confirm the discovery of water locked in lunar soil on the Moon during the Chandrayaan-1 moon rover experiment in 2008-09.

Recipe for optimisation

After confirming the palladium reaction generated water, the researchers sought to optimise the process. They added hydrogen and oxygen separately at different times or mixed together to determine which sequence of events generated water at the fastest rate.

Dravid, Liu and Koo discovered that adding hydrogen first, followed by oxygen, led to the fastest reaction rate. Because hydrogen atoms are so small, they can squeeze between palladium’s atoms, causing the metal to expand. After filling the palladium with hydrogen, the researchers added oxygen gas.

“Oxygen atoms are energetically favourable to adsorb onto palladium surfaces, but they are too large to enter the palladium's square lattice,” Liu said.

“When we flowed in oxygen first, its dissociated atoms covered the entire surface of the palladium, so hydrogen could not adsorb onto the surface to trigger the reaction. But when we stored hydrogen in the palladium first and then added oxygen, the reaction started.

"Hydrogen comes out of the palladium to react with the oxygen, and the palladium shrinks and returns to its initial state.”

Space travel

Although the study focused on bubble generation at nanoscale, the Northwestern team imagines that there could be potential to prepare hydrogen-filled palladium for space travel. Larger sheets of palladium could be used to generate larger quantities of water for drinking or for watering plants, with astronauts only needing to add oxygen once in space.

“Palladium might seem expensive, but it’s recyclable,” Liu said. “Our process doesn’t consume it.

"The only thing consumed is gas, and hydrogen is the most abundant gas in the universe. After the reaction, we can reuse the palladium platform over and over.”