This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Bubbles are round, and we know surface tension does that. But squeeze that gas-liquid boundary into a space only a few tens of nanometers wide—could other forces be at work?
Now, an international team from Kyushu University, Nankai University, Stanford University and the University of Alberta has taken a direct look. Publishing on July 14 in the Journal of the American Chemical Society, they combined three-dimensional transmission electron microscopy (3D TEM) with force analysis to provide evidence for a powerful electric field at nanoconfined air-water interfaces. "Water looks simple, but it's actually incredibly complex," says Qin-Yi Li, associate professor at Kyushu University's Faculty of Engineering.
"Its structure is especially rich at the water-air interface, and it shifts dramatically with scale." A thin liquid film offers a good example—like the one forming the surface of a soap bubble. At everyday scales, it pops at the slightest touch. At the nanoscale, however, water molecules at the interface align like tiny compass needles, forming an ordered, layered structure.
This alignment can generate an electric field, potentially strong enough to stabilize the ultrathin water film and at the same time drive chemical reactions, which may explain why reactions inside micrometer-sized droplets often proceed far faster than in bulk water. Yet the field's strength and even its existence have remained debated. It is invisible, and earlier attempts to detect it, largely through chemical approaches, ran into the same obstacles.
Either the measurement disturbed the field, or the droplet evaporated before it could be captured. Li and collaborators set out to bridge physics and chemistry to overcome both. "If we could directly see the 3D shape of the interface, we could deduce the forces acting on it and provide evidence for the electric field," he explains.
His team did this by sealing air and water inside a carbon nanotube roughly 50 nanometers wide, about 1,000 times thinner than a human hair. This confinement stabilized the interface and suppressed evaporation. Then, using 3D TEM, they reconstructed the interface's full 3D shape.
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