Physical chemistry

Light as a Brake

Light is expected to heat particles up or set them in motion. However, an interdisciplinary team at Ruhr University Bochum, Germany, has now proven the opposite. In aqueous solution, fluorescent carbon nanotubes move much slower once they are irradiated with light. During this process, the diffusion constant decreases with light intensity, an effect linked to direct coupling between electrons in the solid and the molecules of the liquid. The research teams of Sebastian Kruss, Marialore Sulpizi, and Martina Havenith describes the hitherto unknown phenomenon in the science journal Nature from June 10, 2026. “This discovery of light-induced quantum friction fundamentally changes our understanding of interfacial processes,” says Kruss.

Experiment: light as an invisible brake

The nanotubes used in the experiments consist of a carbon mesh and are 100,000 times thinner than a human hair. They fluoresce when irradiated with visible light.

The team observed the movement of the nanotubes under the microscope. Once the tubes were excited via light, they behaved as though the surrounding water had suddenly become more viscous. “Our experiments show that the diffusion decreases when we increase the light intensity,” says Kruss, Professor of Physical Chemistry. “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. This means it is the mobility of the excitons along the nanotube that is in direct exchange with the environment and creates this decelerating effect.”

Theory: understanding the transfer of momentum

Numerical calculations were required in order to understand how an exciton inside a nanotube can decelerate the movement of the entire object in water. To do this, the team used atomistic simulations to make the processes on the interface visible. “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,” explains Professor of Theoretical Physics Marialore Sulpizi. “A tiny but measurable transfer of momentum takes place. The water is not a smooth medium for the illuminated nanotube, but instead there is resistance on the surface that slows the movement.”

Spectroscopy: water as an active partner

One of the key topics that the Excellence Cluster RESOLV (Ruhr Explores Solvation) in Bochum is working on is the fact that water is much more than just a passive solvent. By using terahertz (THz) spectroscopy, the team was able to experimentally demonstrate the immediate coupling between the nanotube and the water.

“With the THz spectroscopy, we were able to determine how the friction and energy dissipation into water occur in real time after excitation of electronic states of the nanotube,” says Professor Martina Havenith, spokeswoman for RESOLV. “It is a textbook example of how solvation interactions with the environment dominate physical properties like friction. This knowledge that we can control the friction at the interface with the liquid via electronic excitation in the solid, opens up entirely new doors in materials science and nanotechnology.”

The discovery of light-induced quantum friction shows that the boundaries between solid physics and liquids blur at the nano-level. Controlling this friction with light offers potential for applications in which transport processes on very small length scales have to be precisely steered.