One hundred thousand times thinner than a human hair, carbon nanotubes were observed slowing down in water not through friction, but through light, in experiments conducted at Ruhr University Bochum. Researchers Sebastian Kruss, Marialore Sulpizi, and Martina Havenith discovered a counterintuitive effect: increasing light intensity actually decreases the nanotubes’ movement, a phenomenon they’ve termed. The team observed that the diffusion constant of the nanotubes decreases with light intensity, linking the effect to the mobility of electronic excitations within the nanotubes and their interaction with surrounding water molecules. “This discovery of light-induced quantum friction fundamentally changes our understanding of interfacial processes,” says Kruss, Professor of Physical Chemistry, suggesting a previously unappreciated role for light in nanoscale interactions.
Quantum Friction Slows Nanotube Diffusion in Aqueous Solution
The slowing effect isn’t simply a matter of light imparting energy; the research indicates a direct coupling between electrons within the nanotubes and the surrounding water molecules. The team found that nanotubes containing defects which impede the movement of excitons, electronic excitations responsible for fluorescence, did not exhibit the same slowing effect. This suggests a direct link between exciton mobility and the observed effect, indicating that the transfer of momentum occurs through these excitations. Atomistic simulations were employed to visualize these interfacial processes, revealing that fluctuating dipole moments of excitons directly interact with collective movements of water molecules, creating measurable resistance.
Further investigation using terahertz spectroscopy confirmed the immediate coupling between the nanotubes and water, demonstrating that water isn’t a passive solvent but an active participant in this interaction. Professor of Theoretical Physics Marialore Sulpizi explains, “By doing so, we were able to show that the fluctuating dipole moments of the excitons in the nanotubes directly couple with the collective movements of the water molecules.” The team’s work highlights the complex interplay between light, quantum phenomena, and solvent behavior at the nanoscale, opening new avenues for exploring interfacial dynamics.
“What’s fascinating is that this effect vanishes entirely when we use nanotubes in which the electronic excitations that lead to the fluorescence – known as excitons – are slowed down at defects.
Terahertz Spectroscopy Reveals Nanotube-Water Coupling & Energy Dissipation
Researchers at Ruhr University Bochum utilized terahertz spectroscopy to observe the immediate coupling between carbon nanotubes and surrounding water molecules, providing experimental evidence for the mechanism behind the observed slowing effect, which builds on observations of light-induced deceleration of nanotubes in aqueous solution. The team, spanning physical chemistry and theoretical physics, demonstrated that water isn’t simply a passive solvent but actively participates in the energy dissipation process, challenging conventional understandings of interfacial interactions. This active role was revealed by tracking how friction and energy transfer into the water occur in real time following light excitation of the nanotubes. The simulations revealed a measurable transfer of momentum, indicating the water molecules create resistance on the nanotube’s surface.
“With the THz spectroscopy, we were able to determine how the friction and energy dissipation into water occur in real time after ex.
