The study of light continues to intrigue researchers, revealing unexpected phenomena, including its sometimes counterintuitive effects. Traditionally, light is known to add energy, causing particles to heat up or move. However, recent findings show light can also act as an invisible brake at incredibly small scales.
A newly published study in Nature by a team from Ruhr-University Bochum in Germany demonstrates that fluorescent carbon-mesh nanotubes slow down when exposed to light in a watery solution.
Increasing light intensity results in slower particle movement or a reduced diffusion constant, which measures particle mobility in a liquid. This phenomenon is partly attributed to ‘quantum friction’, a concept scientists are just beginning to explore.
“The discovery of light-induced quantum friction fundamentally alters our understanding of interfacial processes,” states Sebastian Kruss, a physical chemist at Ruhr-University Bochum. “Our experiments demonstrate that higher light intensity leads to decreased diffusion.”
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The nanotubes, about 100,000 times thinner than a human hair, were individually suspended in water. Microscopic analysis indicated that additional light caused the nanotubes to behave as though they were in a thicker liquid. The aim was to closely examine quantum friction, a drag effect occurring when fluctuating electrical charges in a solid material interact with surrounding liquid molecules.
Under light exposure, as the nanotubes glowed and decelerated, researchers observed the formation of excitons—paired energetic particles made of an electron and a ‘hole’ where an electron used to be—inside the nanotube. These excitons interacted with water molecules, transferring momentum.
“What’s fascinating is that this effect disappears completely when we use nanotubes where the electronic excitations that cause fluorescence—known as excitons—are slowed down at defects,” Kruss notes. “This indicates that the excitons’ mobility along the nanotube directly interacts with the environment, creating this slowing effect.”
To detect molecular-level activity, the researchers used terahertz (THz) spectroscopy, which employs electromagnetic waves to measure molecular energy and motion, particularly energy transfers to water.
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“A tiny but measurable transfer of momentum occurs,” explains theoretical physicist Marialore Sulpizi from Ruhr-University Bochum. “The water does not present a smooth medium for the illuminated nanotube; instead, resistance on the surface slows its movement.”
Quantum friction, unlike traditional friction involving the contact of surfaces, operates at the electron level without physical contact. It’s the fluctuating electrical charges that cause the friction, slowing down the movement of charges within the nanotube as they interact with water molecules. Consequently, light essentially acts as a brake.
These findings also highlight a blurring of lines between solid and liquid physics at the nanoscale, illustrating the phenomenon often termed quantum weirdness.
Related: Scientists Create The Thinnest Lens on Earth Using Quantum Physics
Controlling friction with light could lead to practical applications, such as guiding nanorobots through liquids or altering chemical reaction conditions. “Understanding how we can manage friction at the liquid interface via electronic excitation opens up new possibilities in materials science and nanotechnology,” says Martina Havenith, a physical chemist at Ruhr-University Bochum.
The research is published in Nature.
This article was fact-checked and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
