Light could put the brakes on material diffusing through a solution, and the reason why touches on some of the stranger corners of quantum mechanics.
In a study published in Nature, researchers found that shining light on carbon nanotubes suspended in water slowed the spread of the tiny structures. The more light they applied, the slower the tubes moved. They're calling it light-induced quantum friction.
Quantum friction itself is a strange concept. It’s a drag force that arises between two surfaces, or between a surface and a liquid, as a result of the quantum "noise" that fills the void between objects.
“The phenomenon of quantum friction goes beyond classical mechanics,” said Marialore Sulpizi, theoretical physicist at Ruhr University, Bochum, who led the modeling and simulations of the work, to Refractor. “In quantum friction, this resistance arises from the special behavior of electrons, which obey the laws of quantum mechanics,” she says.
While quantum friction has been studied theoretically for years, observing it in real time has been remarkably difficult. This new study is the clearest experimental evidence yet that friction can emerge not just from surfaces grinding together but also from quantum effects at the interface between matter and a liquid.
"Normally, if you put energy into a system, you would expect it to move faster, not slower," says Sebastian Kruss, a physical chemist at Ruhr University, Bochum, and co-author of the study. "This was a bit counterintuitive."
The team wasn't originally hunting for friction. They work with carbon nanotubes, structures known for their ability to fluoresce in near-infrared light, a property useful for biological imaging.
However, when they illuminated the nanotubes and tracked their random motion in water, something was off: the particles were drifting more slowly than before the light hit them.
“It all started with the experimental observation that carbon nanotubes slow down under illumination. We began discussing the possible physical origin of this phenomenon,” says Sulpizi.
The team also chemically tuned the nanotubes, using compounds that made them glow brighter or dimmer. The results were consistent. Brighter fluorescence meant slower diffusion through the water, while dimmer fluorescence meant faster movement.
The answer came down to how the nanotubes respond to light. When they absorb light, they generate short-lived excited states called excitons. Unlike in most materials, excitons in carbon nanotubes are mobile; they travel along the length of the tube. As they move, they carry fluctuating electrical charges that tug on nearby water molecules, which have their own electric charge imbalance. That interaction creates an additional drag force at the nanotube-water interface, increasing the overall friction.
To confirm the mechanism, the team ran computational simulations and then introduced chemical defects into the nanotubes. The defects trapped the excitons in place, preventing them from moving. Once localized, the light-induced friction vanished entirely.
"Our interpretation is that once the exciton is localized and cannot move anymore, it cannot interact with the water in the same way," Kruss said.
That finding does more than explain the slowdown; it shows that this effect can be switched on and off. Quantum friction, in this case, isn't a fixed material property. It's something that can be tuned.
"This study shows that quantum friction can be induced and controlled by light," said Sulpizi. “This is a new phenomenon that had not been observed before."
The discovery matters beyond the lab bench. Carbon-water interfaces have long puzzled researchers; water behaves unusually when flowing through carbon nanotubes or across graphene surfaces, and quantum effects have been proposed as one explanation. This study gives that theory its most concrete experimental footing yet.
But the work isn’t finished. The team doesn't yet know how the effect varies with different wavelengths of light, or whether similar behaviour appears in other nanomaterials.
In the meantime, the researchers are focused on what the result reveals about the fundamental physics of light, matter, and liquid in close contact.
"It's an experimental proof for a theoretical prediction," Kruss said. "But it also shows that there is a direct interaction between excited states and their environment. Fundamentally, that's quite important."
This research was published in Nature.
Source: Ruhr University, Bochum
Fact-checked by Mike McRae
