Even a single gas molecule can deliver a blow strong enough to be detected. Researchers have just succeeded in measuring it.
All around us, air molecules are constantly moving. They collide with walls, objects, and our skin. These countless impacts produce air pressure, among other things. But each collision considered separately is so weak that it is very difficult to distinguish.
Molecular model of sulfur hexafluoride, with the formula SF₆: a sulfur atom surrounded by six fluorine atoms. This gas was one of the three substances tested in the experiment on individual collisions.
Photo: Ljeto, CC BY-SA 4.0, Wikimedia Commons.
A team from Yale University in the United States has found a way to observe some of these tiny impacts. The researchers used a silica bead, a glass-like material, measuring just 100 nanometers in diameter. It is more than a thousand times smaller than the thickness of a human hair.
To prevent it from touching anything, they suspended it using laser light inside a chamber from which almost all the air had been removed. They then added, in very small quantities, three different gases: krypton, xenon, and sulfur hexafluoride.
With each collision, an atom or molecule could impart momentum to the bead, causing an infinitesimal movement. The laser made it possible to track this displacement. The researchers thus detected impacts produced by particles approximately ten million times lighter than their sensor.
The most difficult part was separating these impacts from all the other small vibrations in the apparatus. The team compared the signals with those recorded without adding gas, and then with the results expected from calculations. The measurements matched the predicted frequency and intensity of the collisions.
The researchers have shown here that it is possible to distinguish individual collisions, whereas instruments usually measure their combined effect.
What could this be used for? By counting these impacts, it might be possible to measure extremely low pressures, such as those found in systems where an almost complete vacuum is created.
With even more sensitive sensors, this method could also help study very weak forces and certain fundamental physics experiments. For now, the achievement lies in the scale of what the researchers managed to measure: the blow of a single gas particle.